U.S. patent number 10,950,799 [Application Number 15/329,851] was granted by the patent office on 2021-03-16 for organic electroluminescent element, display device, lighting device, pi-conjugated compound, and light-emitting thin film.
This patent grant is currently assigned to Merck Patent GmbH. The grantee listed for this patent is Konica Minolta, Inc.. Invention is credited to Hiroshi Kita, Yasuo Miyata, Taketo Namikawa.
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United States Patent |
10,950,799 |
Miyata , et al. |
March 16, 2021 |
Organic electroluminescent element, display device, lighting
device, PI-conjugated compound, and light-emitting thin film
Abstract
An object of the present invention is to provide an organic
electroluminescent element containing an organic layer interposed
between an anode and a cathode, the organic layer containing at
least one light emitting layer, wherein the at least one light
emitting layer contains a .pi.-conjugated compound having an
electron donor portion and an electron acceptor portion in the
molecule; the .pi.-conjugated compound has a direction vector from
an atom having a HOMO orbital in the electron donor portion to an
electron cloud of the HOMO orbital, and a direction vector from an
atom having a LUMO orbital in the electron acceptor portion to an
electron cloud of the LUMO orbital, and the two direction vectors
form an angle .theta. in the range of 90 to 180 degrees; and the
.pi.-conjugated compound has a plurality of the electron donor
portions or a plurality of the electron acceptor portions.
Inventors: |
Miyata; Yasuo (Yokohama,
JP), Namikawa; Taketo (Osaka, JP), Kita;
Hiroshi (Hachioji, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Konica Minolta, Inc. |
Tokyo |
N/A |
JP |
|
|
Assignee: |
Merck Patent GmbH (Darmstadt,
DE)
|
Family
ID: |
1000005426453 |
Appl.
No.: |
15/329,851 |
Filed: |
July 30, 2015 |
PCT
Filed: |
July 30, 2015 |
PCT No.: |
PCT/JP2015/071638 |
371(c)(1),(2),(4) Date: |
January 27, 2017 |
PCT
Pub. No.: |
WO2016/017757 |
PCT
Pub. Date: |
February 04, 2016 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20170271597 A1 |
Sep 21, 2017 |
|
Foreign Application Priority Data
|
|
|
|
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Jul 31, 2014 [JP] |
|
|
JP2014-155852 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
51/0071 (20130101); C07D 487/04 (20130101); C07D
241/48 (20130101); H05B 33/20 (20130101); C07D
403/10 (20130101); C07D 265/38 (20130101); C07F
7/0812 (20130101); C07D 219/02 (20130101); C07D
417/10 (20130101); C07D 209/86 (20130101); C07F
7/10 (20130101); C07D 241/46 (20130101); C09K
11/06 (20130101); C07D 409/10 (20130101); C07F
5/02 (20130101); C07F 5/027 (20130101); C07D
403/12 (20130101); H01L 51/0094 (20130101); C07D
401/10 (20130101); H01L 51/5004 (20130101); H01L
51/0072 (20130101); C07D 307/91 (20130101); C07D
279/22 (20130101); H01L 51/008 (20130101); C07D
413/14 (20130101); C07D 333/76 (20130101); C07D
401/12 (20130101); H01L 2251/552 (20130101); H01L
51/5028 (20130101); H01L 51/0067 (20130101); H01L
51/0085 (20130101); C09K 2211/1022 (20130101); H01L
51/5016 (20130101); H01L 2251/5376 (20130101); H01L
51/0074 (20130101); C09K 2211/1018 (20130101) |
Current International
Class: |
H01L
51/00 (20060101); C07D 487/04 (20060101); C07D
265/38 (20060101); C07D 401/10 (20060101); C07F
7/10 (20060101); C07F 5/02 (20060101); C07D
241/48 (20060101); C07D 219/02 (20060101); C07D
413/14 (20060101); C07F 7/08 (20060101); C07D
401/12 (20060101); C09K 11/06 (20060101); H01L
51/50 (20060101); C07D 333/76 (20060101); C07D
307/91 (20060101); C07D 209/86 (20060101); C07D
279/22 (20060101); C07D 403/10 (20060101); H05B
33/20 (20060101); C07D 417/10 (20060101); C07D
409/10 (20060101); C07D 403/12 (20060101); C07D
241/46 (20060101) |
Field of
Search: |
;428/690 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
103848822 |
|
Jun 2014 |
|
CN |
|
2011-153201 |
|
Aug 2011 |
|
JP |
|
2013116975 |
|
Jun 2013 |
|
JP |
|
2013179291 |
|
Sep 2013 |
|
JP |
|
10-2014-0045368 |
|
Apr 2014 |
|
KR |
|
10-2014-0076521 |
|
Jun 2014 |
|
KR |
|
2005062675 |
|
Jul 2005 |
|
WO |
|
2010134350 |
|
Nov 2010 |
|
WO |
|
2013161437 |
|
Oct 2013 |
|
WO |
|
2014157619 |
|
Oct 2014 |
|
WO |
|
Other References
CAS reg. No. 855828-26-1, Jul. 18, 2005. (Year: 2005). cited by
examiner .
CAS reg. No. 1477512, Nov. 20, 2013. (Year: 2013). cited by
examiner .
CAS reg. No. 1873378-50-7, Feb. 24, 2016. (Year: 2016). cited by
examiner .
English Translation of WO 2005062675, dated Jul. 7, 2005. (Year:
2005). cited by examiner .
IPRP dated Oct. 27, 2015 from corresponding International
Application No. PCT/JP2015/071638; Applicant: Konica Minolta, Inc.;
English translation of IPRP; Total of 11 pages. cited by applicant
.
Office Action dated Dec. 5, 2017 from corresponding Chinese Patent
Application No. 201580041898.X and English translation. cited by
applicant .
Office Action dated May 9, 2018 from the corresponding Korean
Patent Application No. KR 10-2017-7001500 and English translation.
cited by applicant .
Office Action dated Dec. 5, 2017 from corresporading Chinese Patent
Appiication No. 201580041898.X and English transiation cited by
applicant .
Office Action dated Mar. 8, 2019 from the corresponding Korean
Patent Application No. KR10-2017-7001500 and English translation.
cited by applicant .
Office Action dated Nov. 20, 2018 from the corresponding Korean
Patent Application No. 10-2017-7001500 and English translation.
cited by applicant .
H. Uoyama, et al; Nature; vol. 492; 2012; pp. 234-238. cited by
applicant .
Q. Zhang, et al, Nature; Photonics; vol. 8; 2014, pp. 326-332.
cited by applicant .
H. Nakanotani, et al, Nature Communication; vol. 5; 2014, pp.
4016-4022. cited by applicant .
International Search Report dated Oct. 27, 2015 for
PCT/JP2015/071638 and English translation. cited by applicant .
JPO, Office Action for the corresponding Japanese Patent
Application No. 2016-538435, dated Jul. 30, 2019, with English
translation. cited by applicant .
JPO, Office Action for a related Japanese patent application No.
2016-538435, dated Feb. 12, 2020, with English translation (8
pages). cited by applicant .
CNIPA, Office Action for a related Chinese patent application No.
201811293780.8, dated Mar. 30, 2020, with English translation (24
pages). cited by applicant.
|
Primary Examiner: McGinty; Douglas J
Attorney, Agent or Firm: Lucas & Mercanti, LLP
Claims
The invention claimed is:
1. An organic electroluminescent element comprising an organic
layer interposed between an anode and a cathode, the organic layer
containing at least one light emitting layer, wherein the at least
one light emitting layer contains a .pi.-conjugated compound having
an electron donor portion and an electron acceptor portion in the
molecule; the .pi.-conjugated compound has a direction vector from
an atom having a HOMO orbital in the electron donor portion to an
electron cloud of the HOMO orbital, and a direction vector from an
atom having a LUMO orbital in the electron acceptor portion to an
electron cloud of the LUMO orbital, and the two direction vectors
form an angle .theta. in the range of 90 to 180 degrees; and the
.pi.-conjugated compound has a plurality of the electron donor
portions or a plurality of the electron acceptor portions, and the
.pi.-conjugated compound is represented by Formula (3) or Formula
(6): ##STR00029## wherein X.sup.3, X.sup.6, Y.sup.8, Y.sup.9,
Y.sup.17, and Y.sup.18 each respectively represent the electron
donor portion or the electron acceptor portion, X.sup.3, X.sup.6,
Y.sup.8, Y.sup.9, Y.sup.17, and Y.sup.18 each respectively are one
selected from the group consisting of an aryl group which may have
a substituent, a heteroaryl group which may have a substituent, a
carbonyl group which may have a substituent, a nitrogen atom which
may have a substituent, a boron atom which may have a substituent,
a phosphor atom which may have a substituent, and a silicon atom
which may have a substituent, and L.sup.8 and L.sup.9 represent a
linking group, L.sup.8 and L.sup.9 each respectively represent an
aryl group which may have a substituent or a heteroaryl group which
may have a substituent, L.sup.8 binds X.sup.3 and Y.sup.8 through
adjacent carbon atoms, L.sup.9 binds X.sup.3 and Y.sup.9 through
adjacent carbon atoms, and wherein the .pi.-conjugated compound is
not represented by Formula (2): ##STR00030## wherein X.sup.2 and
Y.sup.5 to Y.sup.7 each respectively represent the electron donor
portion or the electron acceptor portion; when X.sup.2 represents
the electron donor portion, Y.sup.5 to Y.sup.7 each respectively
represent the electron acceptor portion; when X.sup.2 represents
the electron acceptor portion, Y.sup.5 to Y.sup.7 each respectively
represent the electron donor portion; L.sup.5 to L.sup.7 each
represent a linking group, L.sup.5 to L.sup.7 each respectively
represent an aryl group which may have a substituent or a
heteroaryl group which may have a substituent, L.sup.5 binds
X.sup.2 and Y.sup.5 through adjacent carbon atoms, L.sup.6 binds
X.sup.2 and Y.sup.6 through adjacent carbon atoms, L.sup.7 binds
X.sup.2 and Y.sup.7 through adjacent carbon atoms.
2. The organic electroluminescent element described in claim 1,
wherein the angle .theta. is in the range of 135 to 180
degrees.
3. The organic electroluminescent element described in claim 1,
wherein one of the electron acceptor portions is bonded to two or
more electron donor portions through the linking group, or one of
the electron donor portions is bonded to two or more electron
acceptor portions through the linking group.
4. The organic electroluminescent element described in claim 1,
wherein one of the electron acceptor portions is directly bonded to
two or more electron donor portions, or one of the electron donor
portions is directly bonded to two or more electron acceptor
portions.
5. The organic electroluminescent element described in claim 1,
wherein L.sup.8 and L.sup.9 in Formula (3) each are a benzene
ring.
6. The organic electroluminescent element described in claim 1,
wherein an absolute value of a difference between a lowest excited
singlet energy level and a lowest triplet energy level
(.DELTA.E.sub.ST) is 0.5 eV or less.
7. The organic electroluminescent element described in claim 1,
wherein the at least one light emitting layer contains: the
.pi.-conjugated compound; and at least one of a fluorescent
compound and a phosphorescent compound.
8. The organic electroluminescent element described in claim 1,
wherein the at least one light emitting layer contains: the
.pi.-conjugated compound; at least one of a fluorescent compound
and a phosphorescent compound; and a host compound.
9. A display device provided with the organic electroluminescent
element described in claim 1.
10. A lighting device provided with the organic electroluminescent
element described in claim 1.
11. A .pi.-conjugated compound having an electron donor portion and
an electron acceptor portion in the molecule, wherein the
.pi.-conjugated compound has a direction vector from an atom having
a HOMO orbital in the electron donor portion to an electron cloud
of the HOMO orbital, and a direction vector from an atom having a
LUMO orbital in the electron acceptor portion to an electron cloud
of the LUMO orbital, and the two direction vectors form an angle
.theta. in the range of 90 to 180 degrees; and the .pi.-conjugated
compound has a plurality of the electron donor portions or a
plurality of the electron acceptor portions, and the
.pi.-conjugated compound is represented by Formula (3) or Formula
(6): ##STR00031## wherein X.sup.3, X.sup.6, Y.sup.8, Y.sup.9,
Y.sup.17, and Y.sup.18 each respectively represent the electron
donor portion or the electron acceptor portion, and X.sup.3,
X.sup.6, Y.sup.8, Y.sup.9, Y.sup.17, and Y.sup.18 each respectively
are one selected from the group consisting of an aryl group which
may have a substituent, a heteroaryl group which may have a
substituent, a carbonyl group which may have a substituent, a
nitrogen atom which may have a substituent, a boron atom which may
have a substituent, a phosphor atom which may have a substituent,
and a silicon atom which may have a substituent, and L.sup.8 and
L.sup.9 represent a linking group, L.sup.8 and L.sup.9 each
respectively represent an aryl group which may have a substituent
or a heteroaryl group which may have a substituent, L.sup.8 binds
X.sup.3 and Y.sup.8 through adjacent carbon atoms, L.sup.9 binds
X.sup.3 and Y.sup.9 through adjacent carbon atoms, and wherein the
.pi.-conjugated compound is not represented by Formula (2):
##STR00032## wherein X.sup.2 and Y.sup.5 to Y.sup.7 each
respectively represent the electron donor portion or the electron
acceptor portion; when X.sup.2 represents the electron donor
portion, Y.sup.5 to Y.sup.7 each respectively represent the
electron acceptor portion; when X.sup.2 represents the electron
acceptor portion, Y.sup.5 to Y.sup.7 each respectively represent
the electron donor portion; L.sup.5 to L.sup.7 represent a linking
group, L.sup.5 to L.sup.7 each respectively represent an aryl group
which may have a substituent or a heteroaryl group which may have a
substituent, L.sup.5 binds X.sup.2 and Y.sup.5 through adjacent
carbon atoms, L.sup.6 binds X.sup.2 and Y.sup.6 through adjacent
carbon atoms, L.sup.7 binds X.sup.2 and Y.sup.7 through adjacent
carbon atoms.
12. A light-emitting thin film containing the .pi.-conjugated
compound described in claim 11.
Description
CROSS REFERENCE TO RELATED APPLICATION
This Application is a 371 of PCT/JP2015/071638 filed on Jul. 30,
2015, which, in turn, claimed the priority of Japanese Patent
Application No. JP 2014-155852 filed on Jul. 31, 2014, both
applications are incorporated herein by reference.
TECHNICAL FIELD
The present invention relates to an organic electroluminescent
element. Further, the present invention relates to a display device
and a lighting device provided with the organic electroluminescent
element, a .pi.-conjugated compound and a light-emitting thin film
containing the .pi.-conjugated compound. More specifically, the
present invention relates to an organic electroluminescent element
achieving improved light emitting efficiency.
BACKGROUND
Organic electroluminescent (hereinafter referred to as "EL")
elements (also referred to as "organic electroluminescence
elements"), which are based on electroluminescence of organic
materials, have already been put into practice as a new generation
of light emitting systems capable of achieving planar light
emission. Organic EL elements have recently been applied to
electronic displays and also to lighting devices and display
devices. Thus, it has been demanded further development of organic
EL elements.
As an emission mode of an organic EL, there are two types. One is
"a phosphorescence emission type" which emits light when a triplet
excited state returns to a ground state, and another one is "a
fluorescence emission type" which emits light when a singlet
excited state returns to a ground state.
When an electric filed is applied to an organic EL element, a hole
and an electron are respectively injected from an anode and a
cathode, they are recombined in a light emitting layer to produce
an exciton. At this moment, a singlet exciton and a triplet exciton
are formed with a ratio of 25%:75%. Therefore, it is known that a
phosphorescence emission type using a triplet exciton will produce
theoretically high internal quantum efficiency compared with a
fluorescence emission type.
However, in order to obtain high quantum efficiency in a
phosphorescence emission type, it is required to use a complex
compound having a rare metal of iridium or platinum in the center
metal. This may induce an industrial problem of the amount of
deposits or the cost of the rare metals in the future.
On the other hand, in recent years, new techniques relevant to a
fluorescence emission type have been proposed to improve emission
efficiency.
For example, Patent document 1 discloses a technique which is
focused on a phenomenon wherein singlet excitons are generated by
collision of two triplet excitons (it is called as Triplet-Triplet
Annihilation (TTA), or Triplet-Triplet Fusion (TTF)), and which
improves the emission efficiency of a fluorescent element by
allowing the TTA phenomenon to occur effectively. Although this
technique can increase power efficiency of a fluorescence emission
material (hereafter, it is called as a fluorescent emission
material or fluorescent material) from two to three times larger
than the power efficiency of a conventional fluorescent material,
the emission efficiency in TTA is not as high as that of the
aforementioned phosphorescent material due to a theoretical
limitation, because the rate of conversion of the excited triplet
energy level to the excited singlet energy level will remain to
about 40%.
Recent studies have disclosed a fluorescent material that employs a
thermally activated delayed fluorescent mechanism (hereinafter also
referred to as "TADF"). It is reported that it may be applied to an
organic EL element (for example, refer to Patent document 2 and
Non-patent documents 1 to 2). By making use of this delayed
fluorescence caused by the TADF mechanism, theoretically, it is
possible to achieve an internal quantum efficiency of 100% in
fluorescence emission, which is similar to the phosphorescent
emission.
In order to make appear the TADF phenomenon, it is required that a
reverse intersystem crossing from the triplet state, which is
produced with an amount of 75% by an electric field excitation in
an amount of 75% at room temperature or at an emission layer
temperature on the emission device, to the singlet state should be
taken place. Further, by the mechanism that the singlet exciton
produced by the reverse intersystem crossing emits fluorescence in
the same way as the singlet exciton produced with an amount of 25%,
it is theoretically possible to realize 100% internal quantum
efficiency. In order to make appear this reverse intersystem
crossing, it is necessary that the absolute value of the difference
between the singlet excited level and the triplet excited level
(hereafter, it is called as .DELTA.E.sub.ST) is very small. To
obtain a minimum .DELTA.E.sub.ST in an organic compound, it is
preferable that a HOMO and a LUMO in the molecule are not mixed and
localized respectively.
For example, in the case of 2CzPN illustrated in "a" of FIG. 1, a
HOMO is localized at a carbazolyl group at the 1 position and the 2
position of the benzene ring, and a LUMO is localized at cyano
groups at the 4 position and the 5 position. As a result, the HOMO
and the LUMO of 2CzPN may be separated, and .DELTA.E.sub.ST becomes
very small as indicated in "b" of FIG. 1. Thus a TADF phenomenon
will be produced. On the other hand, in the case of 2CzXy ("a" of
FIG. 2) which is produced by substituting cyano groups at the 4
position and the 5 position of 2CzPN with methyl groups, the HOMO
and the LUMO cannot be clearly separated as is seen in 2CzPN. As a
result, .DELTA.E.sub.ST cannot be made small, and a TADF phenomenon
will not be produced.
Further, it is known that an addition of the third component (an
assist-dopant compound) which exhibits a TADF property into a light
emitting layer composed of a host compound and an emission compound
is effective to achieve high efficiency (Non-patent document 3). By
producing 25% of singlet exciton and 75% of triplet exciton via an
electric field excitation on an assist-dopant compound, the triplet
exciton will produce the singlet exciton through the reverse
intersystem crossing (RISC). The energy of the singlet exciton will
be moved to the emission compound via an energy transfer. It is
possible that the emission compound emits light. Consequently,
theoretically, it is possible to emit light from the emission
compound by making use of 100% of the exciton. It may achieve high
emission efficiency.
However, the localization of the HOMO and the LUMO, which is a
requirement for making appear the TADF phenomenon, will form an
excited state having an intermolecular charge transfer (CT)
property. This will become a factor of broadening an absorption
spectrum or an emission spectrum. This broadening phenomenon
becomes a fatal problem for a color designing of an organic EL
element. The reason of this problem will be described in the
following,
An electronic state of 2CzPN is schematically illustrated in "a" of
FIG. 3. A molecule known as a TADF emission material (hereafter, it
may be called as "a TADF compound") has a localized HOMO and a
localized LUMO, and it has an imbalanced charge in the molecule.
This charge imbalance will induce imbalance in the medium substance
(for example, a solvent or a host compound, see "b" of FIG. 3).
Therefore, as indicated in "c" of FIG. 3, the medium substance will
be electrostatically adsorbed to the TADF compound. Interactions
will be formed at a variety of positions and directions. As a
result, an energy distribution in the excited state of the TADF
compound will be spread, and it is known that an absorption
spectrum or an emission spectrum will be broadened.
On the other hand, FIG. 4 illustrates a schematic diagram of an
interaction between a phosphorescent compound and a host compound.
As illustrated in "a" of FIG. 4, the phosphorescent compound
(Ir(ppy).sub.3) has a localized HOMO and a localized LUMO
respectively placed in the inner portion and in the outer portion
of the molecule. Since the HOMO portion exists substantially at an
iridium metal in the center of the complex, it will not contribute
to an electrostatic interaction with the surrounding medium. The
LUMO distributed in the ligand will interact with the host compound
("b" of FIG. 4). Since it is localized in the outer side of the
molecule, the location and direction of the interaction will be
limited. Consequently, an energy distribution in the excited state
of the phosphorescent compound will be restrained, and broadening
of an absorption spectrum and an emission spectrum will become
small compared with a conventional TADF compound ("c" of FIG.
4).
Accordingly, it is required a new molecular design for a TADF
compound enabling to restrain an energy distribution in the excited
state as the phosphorescent compound. A conventional organic EL
element has not achieved all of the following properties at the
same time: restrain of broadening of an absorption spectrum and an
emission spectrum, high emission efficiency, and non-use of a rare
metal.
PRIOR ART DOCUMENTS
Patent Documents
Patent document 1: WO 2010/134350
Patent document 2: JP-A No. 2013-116975
Non-Patent Documents
Non-patent document 1: I. Uoyama, et al., Nature, 2012, 492,
234-238.
Non-patent document 2: Q. Mang et al., Nature, Photonics, 2014, 8,
326--332.
Non-patent document 3: H. Nakanotani, et al., Nature Communication,
2014, 5, 4016-4022.
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
The present invention has been made in view of the above-described
problems and situation. An object of the present invention is to
provide an organic electroluminescent element enabling to achieve
restrained broadening of an absorption spectrum and an emission
spectrum, and high emission efficiency without using a rare metal.
An object of the present invention is to provide a display device
and a lighting device provided with the organic electroluminescent
element. Further, an object of the present invention is to provide
a .pi.-conjugated compound enabling to achieve restrained
broadening of an absorption spectrum and an emission spectrum, and
high emission efficiency without using a rare metal, and a
light-emitting thin film containing the .pi.-conjugated
compound.
Means to Solve the Problems
The present inventors have investigated the cause of the
above-described problems in order to solve the problems. It was
found to provide an organic EL element capable of improving
emission efficiency by incorporating a specific .pi.-conjugated
compound having a donor portion and an acceptor portion each being
placed in a specific positional relationship in the molecule in at
least one of the light-emitting layer.
That is, the above-described problems of the present invention are
solved by the following embodiments.
1. An organic electroluminescent element comprising an organic
layer interposed between an anode and a cathode, the organic layer
containing at least one light emitting layer,
wherein the at least one light emitting layer contains a
.pi.-conjugated compound having an electron donor portion and an
electron acceptor portion in the molecule;
the .pi.-conjugated compound has a direction vector from an atom
having a HOMO orbital in the electron donor portion to an electron
cloud of the HOMO orbital, and a direction vector from an atom
having a LUMO orbital in the electron acceptor portion to an
electron cloud of the LUMO orbital, and the two direction vectors
form an angle .theta. in the range of 90 to 180 degrees; and the
.pi.-conjugated compound has at least one of a plurality of the
electron donor portions and a plurality of the electron acceptor
portions.
2. The organic electroluminescent element described in the
embodiment 1,
wherein the angle .theta. is in the range of 135 to 180
degrees.
3. The organic electroluminescent element described in the
embodiments 1 or 2,
wherein one of the electron acceptor portions is bonded to two or
more electron donor portions through a linking group, or one of the
electron donor portions is bonded to two or more electron acceptor
portions through a linking group.
4. The organic electroluminescent element described in the
embodiments 1 or 2,
wherein one of the electron acceptor portions is directly bonded to
two or more electron donor portions, or one of the electron donor
portions is directly bonded to two or more electron acceptor
portions.
5. The organic electroluminescent element described in any one of
the embodiments 1 to 4,
wherein the at least one light emitting layer contains a
.pi.-conjugated compound represented by any one of Formulas (1) to
(8).
##STR00001##
In Formulas, X.sup.1 to X.sup.8 and Y.sup.1 to Y.sup.20 each
respectively represent the electron donor portion or the electron
acceptor portion; when X.sup.1 to X.sup.8 each respectively
represent the electron donor portion, Y.sup.1 to Y.sup.20 each
respectively represent the electron acceptor portion; when X.sup.1
to X.sup.8 each respectively represent the electron acceptor
portion, Y.sup.1 to Y.sup.20 each respectively represent the
electron donor portion; L.sup.1 to L.sup.10 represent a linking
group, L.sup.1 to L.sup.10 each respectively represent an aryl
group which may have a substituent or a heteroaryl group which may
have a substituent, L.sup.1 binds X.sup.1 and Y.sup.1 through
adjacent carbon atoms, L.sup.2 binds X.sup.1 and Y.sup.2 through
adjacent carbon atoms, L.sup.3 binds X.sup.1 and Y.sup.3 through
adjacent carbon atoms, L.sup.4 binds X.sup.1 and Y.sup.4 through
adjacent carbon atoms, L.sup.5 binds X.sup.2 and Y.sup.5 through
adjacent carbon atoms, L.sup.6 binds X.sup.2 and Y.sup.6 through
adjacent carbon atoms, L.sup.7 binds X.sup.2 and Y.sup.7 through
adjacent carbon atoms, L.sup.8 binds X.sup.3 and Y.sup.8 through
adjacent carbon atoms, L.sup.9 binds X.sup.3 and Y.sup.9 through
adjacent carbon atoms, and L.sup.10 binds X.sup.7 and Y.sup.19
through adjacent carbon atoms.
6. The organic electroluminescent element described in the
embodiment 5, wherein the electron donor portion and the electron
acceptor portion represented by to X.sup.8 and to Y.sup.20 in
Formulas (1) to (8) each respectively are one selected from the
group consisting of an aryl group which may have a substituent, a
heteroaryl group which may have a substituent, an alkyl group which
may have a substituent, a carbonyl group which may have a
substituent, a nitrogen atom which may have a substituent, a sulfur
atom which may have a substituent, a boron atom which may have a
substituent, a phosphor atom which may have a substituent, an
oxygen atom which may have a substituent, and a silicon atom which
may have a substituent. 7. The organic electroluminescent element
described in the embodiments 5 or 6,
wherein L.sup.1 to L.sup.10 in Formulas (1) to (3) and (7) each are
a benzene ring.
8. The organic electroluminescent element described in any one of
the embodiments 1 to 7,
wherein an absolute value of a difference between a lowest excited
singlet energy level and a lowest excited triplet energy level
(.DELTA.E.sub.ST) is 0.5 eV or less.
9. The organic electroluminescent element described in any one of
the embodiments 1 to 8,
wherein the at least one light emitting layer contains: the
.pi.-conjugated compound; and at least one of a fluorescent
compound and a phosphorescent compound.
10. The organic electroluminescent element described in any one of
the embodiments 1 to 9,
wherein the at least one light emitting layer contains: the
.pi.-conjugated compound; at least one of a fluorescent compound
and a phosphorescent compound; and a host compound.
11. A display device provided with the organic electroluminescent
element described in any one of the embodiments 1 to 10.
12. A lighting device provided with the organic electroluminescent
element described in any one of the embodiments 1 to 10.
13. A .pi.-conjugated compound having an electron donor portion and
an electron acceptor portion in the molecule,
wherein the .pi.-conjugated compound has a direction vector from an
atom having a HOMO orbital in the electron donor portion to an
electron cloud of the HOMO orbital, and a direction vector from an
atom having a LUMO orbital in the electron acceptor portion to an
electron cloud of the LUMO orbital, and the two direction vectors
form an angle .theta. in the range of 90 to 180 degrees; and the
.pi.-conjugated compound has a plurality of the electron donor
portions or a plurality of the electron acceptor portions.
14. A light-emitting thin film containing the .pi.-conjugated
compound described in the embodiment 13.
Effects of the Invention
By the above-described embodiments of the present invention, it is
possible to provide an organic electroluminescent element enabling
to achieve restrained broadening of an absorption spectrum and an
emission spectrum, and high emission efficiency without using a
rare metal. It is also possible to provide a display device and a
lighting device provided with the organic electroluminescent
element. Further, it is possible to provide a .pi.-conjugated
compound enabling to achieve restrained broadening of an absorption
spectrum and an emission spectrum, and high emission efficiency
without using a rare metal, and a light-emitting thin film
containing the .pi.-conjugated compound.
A formation mechanism or an action mechanism of the effects of the
present invention is not clearly identified, but it is supposed as
follows.
The present invention is specifically effective when the
above-described angle .theta. is in the range of 90 to 180
degrees.
In FIG. 5, "a-1" to "a-3" and "b" each are a schematic drawing
illustrating a .pi.-conjugated compound containing one electron
donor portion and one electron acceptor portion for convenience. It
will be described the case in which the angle .theta. formed with a
direction vector of a donor portion and a direction vector of an
acceptor portion is in the range of the present invention, and the
case in which the angle .theta. is outside the range of the present
invention by making use of these "a-1" to "a-3" and "b" in FIG. 5.
Here, arrows in "a-1" to "a-3" and "b" of FIG. 5 each represent: a
direction vector from an atom having a HOMO orbital in the electron
donor portion to an electron cloud of the HOMO orbital; or a
direction vector from an atom having a LUMO orbital in the electron
acceptor portion to an electron cloud of the LUMO orbital.
In "a-1" to "a-3" of FIG. 5 indicating the angle .theta. in the
range of the present invention, an electron transfer in space will
easily occur from a HOMO of the donor portion to a LUMO of the
acceptor portion. As a result, high emission efficiency will be
achieved in an organic EL element. On the other hand, when the
angle .theta. is outside the range of the present invention as
illustrated in "b" of FIG. 5, an electron transfer in space will
hardly occur from the HOMO of the donor portion to the LUMO of the
acceptor portion. As a result, high emission efficiency will not be
achieved.
In the following, it will be described the restraining effect of
broadening of an absorption spectrum or an emission spectrum. In
"a" of FIG. 6, there is illustrated for convenience a simplified
schematic drawing of a .pi.-conjugated compound containing one
electron donor portion and two electron acceptor portions, and
having an angle .theta. within the range of the present invention.
A HOMO distributed in a donor portion of a TADF compound is facing
to a LUMO in an acceptor portion. There is no space where the HOMO
will interact with the surrounding medium. Consequently, as
illustrated in "b" of FIG. 6, the position and the direction of the
interaction between the .pi.-conjugated compound (the TADF
molecule) of the present invention and the medium (the host
compound) will be limited compared with the case of "c" in FIG. 3.
As a result, it is produced a restraining effect of broadening of
an absorption spectrum or an emission spectrum.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an energy diagram illustrating .DELTA.E.sub.ST.
FIG. 2 is another example of an energy diagram illustrating
.DELTA.E.sub.ST.
FIG. 3 is a schematic interaction diagram of a TADF compound and a
host compound.
FIG. 4 is a schematic diagram illustrating an interaction of a
phosphorescent compound and a host compound.
FIG. 5 is a schematic diagram illustrating an angle .theta. of the
present invention.
FIG. 6 is a schematic diagram illustrating an interaction of a
.pi.-conjugated compound of the present invention and a medium.
FIG. 7 is a schematic diagram illustrating the case in which a
.pi.-conjugated compound has a function of an assist-dopant.
FIG. 8 is a schematic diagram illustrating the case in which a
.pi.-conjugated compound has a function of a host compound.
FIG. 9 is a schematic diagram illustrating an example of a display
device including an organic EL element.
FIG. 10 is a schematic diagram of a display device by an active
matrix mode.
FIG. 11 is a schematic view illustrating a pixel circuit.
FIG. 12 is a schematic diagram of a display device by a passive
matrix mode.
FIG. 13 is a schematic view of a lighting device.
FIG. 14 is a cross-sectional diagram of a lighting device.
FIG. 15 is a schematic diagram illustrating a calculation method of
an angle .theta. of the present invention.
EMBODIMENTS TO CARRY OUT THE INVENTION
An organic electroluminescent element of the present invention is
characterized in having the following features. It comprises an
organic layer interposed between an anode and a cathode, the
organic layer containing at least one light emitting layer, wherein
the at least one light emitting layer contains a .pi.-conjugated
compound having an electron donor portion and an electron acceptor
portion in the molecule; the .pi.-conjugated compound has a
direction vector from an atom having a HOMO orbital in the electron
donor portion to an electron cloud of the HOMO orbital, and a
direction vector from an atom having a LUMO orbital in the electron
acceptor portion to an electron cloud of the LUMO orbital, and the
two direction vectors form an angle .theta. in the range of 90 to
180 degrees; and the .pi.-conjugated compound has a plurality of
the electron donor portions or a plurality of the electron acceptor
portions.
The above-described features are technical features commonly owned
by the invention according to the embodiments 1 to 14.
From the viewpoint of obtaining an effect of the present invention,
a preferable embodiment of the present invention is that the
above-described angle .theta. is in the range of 135 to 180
degrees. By this embodiment, an electron transfer in space will
easily occur from the donor portion to the acceptor portion. As a
result, emission efficiency will be further improved. This is a
preferable embodiment.
A preferable embodiment of the present invention is that one of the
electron acceptor portions is bonded to two or more electron donor
portions through a linking group, or one of the electron donor
portions is bonded to two or more electron acceptor portions
through a linking group. This embodiment will restrain broadening
of an absorption spectrum or an emission spectrum.
Another preferable embodiment of the present invention is that one
of the electron acceptor portions is directly bonded to two or more
electron donor portions, or one of the electron donor portions is
directly bonded to two or more electron acceptor portions. This
embodiment will restrain broadening of an absorption spectrum or an
emission spectrum.
Another preferable embodiment of the present invention is that the
at least one light emitting layer contains a .pi.-conjugated
compound represented by any one of Formulas (1) to (8). This
embodiment is preferable from the viewpoint of achieving high
emission efficiency.
Another preferable embodiment of the present invention is that the
electron donor portion and the electron acceptor portion
represented by X.sup.1 to X.sup.8 and Y.sup.1 to Y.sup.20 in
Formulas (1) to (8) each respectively are one selected from the
group consisting of an aryl group which may have a substituent, a
heteroaryl group which may have a substituent, an alkyl group which
may have a substituent, a carbonyl group which may have a
substituent, a nitrogen atom which may have a substituent, a sulfur
atom which may have a substituent, a boron atom which may have a
substituent, a phosphor atom which may have a substituent, an
oxygen atom which may have a substituent, and a silicon atom which
may have a substituent. This embodiment is preferable from the
viewpoint of achieving high emission efficiency.
Another preferable embodiment of the present invention is that
L.sup.1 to L.sup.10 in Formulas (1) to (3) and (7) each are a
benzene ring. This embodiment is preferable from the viewpoint of
achieving high emission efficiency.
Another preferable embodiment of the present invention is that an
absolute value of a difference between a lowest excited singlet
energy level and a lowest excited triplet energy level
(.DELTA.E.sub.ST) is 0.5 eV or less. This embodiment is preferable
from the viewpoint of easily achieving an intersystem crossing.
Another preferable embodiment of the present invention is that the
at least one light emitting layer contains: the .pi.-conjugated
compound; and at least one of a fluorescent compound and a
phosphorescent compound. This embodiment is preferable from the
viewpoint of achieving high emission efficiency.
Another preferable embodiment of the present invention is that the
at least one light emitting layer contains: the .pi.-conjugated
compound; at least one of a fluorescent compound and a
phosphorescent compound; and a host compound. This embodiment is
preferable from the viewpoint of achieving high emission
efficiency.
An organic electroluminescent element of the present invention is
suitably incorporated in a display device. This is a preferable
embodiment because a display device having high emission efficiency
will be obtained.
An organic electroluminescent element of the present invention is
suitably incorporated in a lighting device. This is a preferable
embodiment because a lighting device having high emission
efficiency will be obtained.
A .pi.-conjugated compound of the present invention is
characterized in having an electron donor portion and an electron
acceptor portion in the molecule, wherein the .pi.-conjugated
compound has a direction vector from an atom having a HOMO orbital
in the electron donor portion to an electron cloud of the HOMO
orbital, and a direction vector from an atom having a LUMO orbital
in the electron acceptor portion to an electron cloud of the LUMO
orbital, and the two direction vectors form an angle .theta. in the
range of 90 to 180 degrees; and the .pi.-conjugated compound has at
least one of a plurality of the electron donor portions and a
plurality of the electron acceptor portions. It may be provided a
material having high emission efficiency by this embodiment.
A .pi.-conjugated compound of the present invention may be suitably
incorporated in a light-emitting thin film. It will be obtained a
light-emitting thin film having high efficiency by this
embodiment.
The present invention and the constitution elements thereof, as
well as configurations and embodiments, will be detailed in the
following. In the present description, when two figures are used to
indicate a range of value before and after "to", these figures are
included in the range as a lowest limit value and an upper limit
value.
<Light Emission Mode of Organic EL>
As a light emission mode of an organic EL, there are two types. One
is "a phosphorescence emission type" which emits light when a
triplet excited state returns to a ground state, and another one is
"a fluorescence emission type" which emits light when a singlet
excited state returns to a ground state.
When excitation is done by an electric field such as in the case of
an organic EL element, a triplet exciton is produced with a
probability of 75%, and a singlet exciton is produced with a
probability of 25%. Consequently, it is possible that a
phosphorescent emission has higher emission efficiency than
fluorescent emission. The phosphorescent emission is an excellent
mode to realize low electric consumption.
On the other hand, with respect to the fluorescent emission, it was
found a method of using a TTA mechanism in which singlet excitons
are generated from two triplet excitons (it is called as
Triplet-Triplet Annihilation (TTA), or Triplet-Triplet Fusion
(TTF)) to improve the emission efficiency. The TTA mechanism may be
achieved by the triplet excitons produced with a probability of
75%, which will normally take the route of radiationless
deactivation only to produce heat. By making the triplet excitons
to be produced in a high density, the TTA mechanism is
effective.
In recent years, the group of Adachi found the following
phenomenon. By achieving a small energy gap between the singlet
excited state and the triplet excited state, it is allowed to occur
a reverse intersystem crossing from the triplet state of lower
energy level to the singlet state. This may be done by the Joule
heat produced during the emission and/or the environmental
temperature in which the light emission element is placed. As a
result, it may be achieved a fluorescent emission in a yield of
nearly 100% (it is called as a thermally activated delayed
fluorescence: TADF). And it was found a compound enabling to occur
this phenomenon (refer to Non-patent document 1, for example).
<Phosphorescence Emission Material>
As described above, although the phosphorescence emission has
theoretically an advantage of 3 times of the fluorescence emission,
an energy deactivation (=phosphorescence emission) from the triplet
excited state to the singlet ground state is a forbidden
transition. In the same manner, the intersystem crossing from the
singlet excited state to the triplet excited state is also a
forbidden transition. Consequently, its rate constant is usually
small. That is, since the transition takes place hardly, the
lifetime of the exciton becomes long such as an order of
millisecond or second. As a result, it is difficult to obtain a
required emission.
However, when an emission occurs from a complex including a heavy
atom of iridium or platinum, the rate constant of the
above-described forbidden transition becomes larger by 3 orders due
to the heavy metal effect of the center metal. It is possible to
obtain a phosphorescence quantum efficiency of 100% when selection
of the ligand is properly done.
However, in order to obtain an ideal emission, it is required to
use a rare metal such as iridium or palladium, or a noble metal
such as platinum. If a large amount of these metals are used, the
reserves and the price of these metal will become problem.
<Fluorescence Emission Material>
A common fluorescence emission material is not required to be a
heavy metal complex as in the case of a phosphorescence emission
material. It may be applied a so-called organic compound composed
of a combination of elements such as carbon, oxygen, nitrogen and
hydrogen. Further, a non-metallic element such as phosphor, sulfur,
and silicon may be used. And a complex of typical element such as
aluminum or zinc may be used. The variation of the materials is
almost without limitation.
However, the conventional fluorescence emission material will use
only 25% of the excitons to light emission. Therefore, it cannot be
expected high emission efficiency as achieved in phosphorescence
emission.
<Delayed Fluorescent Material>
[Excited Triplet-Triplet Annihilation (TTA) Delayed Fluorescent
Material]
A light emission mode employing a delayed fluorescence appeared to
solve the problem of the fluorescent material. The TTA mode
originated from the collision of the compounds at a triplet state
may be described in the following Scheme. That is, in the past, a
part of the triplet exciton is only converted to heat. This energy
of the exciton is changed to a singlet exciton via an intersystem
crossing to result in contributing to the light emission. In a
practical organic EL element, it was proved that external quantum
efficiency was double of the conventional fluorescent element.
ti Scheme: T*+T*.fwdarw.S*+S
(In the Scheme, T* represents a triplet exciton, S* represents a
singlet exciton, and S represents a ground state molecule.)
However, as can be seen from the above-described Scheme, only one
singlet exciton is generated from two triplet excitons.
Consequently, theoretically, 100% internal quantum efficiency
cannot be obtained based on this mode.
[Thermally Activated Delayed Fluorescent (TADF) Compound]
A TADF mode, which is another type of high efficient fluorescence
emission, is a mode enabling to resolve the problem.
A fluorescent material has an advantage of being molecular-designed
without imitation as described above. Among the molecular-designed
compounds, there are specific compounds having an energy level
difference (hereafter, it is indicated as .DELTA.E.sub.ST) between
a triplet excited state and a singlet excited state being in very
close vicinity (refer to "a" in FIG. 1).
In spite of that fact that these compounds don't contain a heavy
metal atom in the molecule, there occurs a reverse intersystem
crossing reaction from the triplet excited state to the singlet
excited state due to the small .DELTA.E.sub.ST value. This reaction
will not usually occur. Further, since the rate constant of the
deactivation from the singlet excited state to the ground state
(=fluorescence emission) is extremely high, the triplet state will
likely return to the ground state via the singlet state while
emitting fluorescence, instead of thermally deactivating
(radiationless deactivation) to the ground state. As a result, in
TADF mechanism, ideally, it is possible to realize fluorescence
emission of 100%.
<Molecular Designing Idea Concerning .DELTA.E.sub.ST>
A molecular designing idea to reduce the .DELTA.E.sub.ST will be
described.
In order to reduce the value of .DELTA.E.sub.ST, theoretically the
most effective way is to minimize the spatial overlaps of the
highest occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO).
Generally, in the electronic orbitals of the molecule, it is known
that HOMO has a distribution to an electron donating position and
LUMO has a distribution to an electron withdrawing position. By
introducing an electron donating structure and an electron
withdrawing structure in the molecule, it is possible to keep apart
the positions in which HOMO and LUMO exist.
For example, Applied Physics vol. 82, no. 6, 2013 "Organic
Photo-electronics in the commercialization stage" discloses the
following. By introducing an electron withdrawing structure such as
a cyano group, a sulfonyl group or a triazine group, and an
electron donating structure such as a carbazole group or a diphenyl
amino group, LUMO and HOMO are respectively made localized.
In addition, it is also effective to minimize the molecular
structure change between the ground state and the triplet excited
state of the molecule. As a means to minimize the structure change,
it can cite a compound having an inflexible structure. Here,
inflexibility indicates the state in which freely movable portions
in the molecule are not abundant caused by preventing a free
rotation of the bond between the rings in the molecule, or by
introducing a condensed ring having a large n-conjugate plane, for
example. In particular, by making the portion participating in the
light emission to be rigid, it is possible to minimize the
molecular structure change in the excited state.
<Common Problem Possessed by TADF Compound>
A TADF compound possesses a variety of problems arisen from the
aspects of the light emission mechanism and the molecular
structure.
A part of common problems possessed by a TADF compound will be
described in the following.
In a TADF compound, it is required to keep apart the portions in
which HOMO and LUMO exist as much as possible in order to minimize
.DELTA.E.sub.ST. For this reason, the electronic state of the
molecule becomes almost near the intra molecular CT state
(intramolecular charge transfer state).
When a plurality of these molecules exist, these molecules will be
stabilized by making in proximity the donor portion in one molecule
and the acceptor portion in other molecule. This stabilized
condition is formed not only with 2 molecules, but it may be formed
with 3 and 5 molecules. Consequently, there are produced a variety
of stabilized conditions having a broad distribution. The shape of
absorption spectrum or the emission spectrum will be broad.
Further, even if a multiple molecular aggregation of 2 or more
molecules does not formed, there may be formed a variety of
existing conditions having different interaction directions or
angles of two molecules. As a result, basically, the shape of
absorption spectrum or the emission spectrum will be broad.
When the emission spectrum becomes broad, it will generate two
major problems. One is a problem of decreasing the color purity of
the emission color. This is not so important when it is applied to
an illumination use. However, when it is used for an electronic
device, the color reproduction region becomes small. And the color
reproduction of pure colors will become decreased. As a result, it
is difficult to apply to a commercial product.
Another problem is the shortened wavelength of the rising
wavelength in the short wavelength side of the emission spectrum
(it is called as "fluorescent zero-zero band"). That is, the
S.sub.1 level becomes high (becoming higher energy level of the
excited singlet energy).
When the fluorescent zero-zero band becomes shortened, the
phosphorescent zero-zero band derived from T.sub.1 (being lower
than S.sub.1) will become shortened (becoming higher T.sub.1).
Therefore, the host compound is required to have high S.sub.1 and
high T.sub.1 in order to prevent the reverse energy transfer from
the dopant.
This is a major problem. A host compound basically made of an
organic compound will take plural and unstable chemical species
conditions such as a cationic radical state, an anionic radical
state and an excited state in an organic EL element. These chemical
species may be made existed in relatively stable condition by
expanding a .pi.-conjugate system in the molecule.
Further, in a TADF compound without containing a heavy metal, the
transition from the triplet excited state to the ground state is
forbidden transition. The existing time at the triplet excited
state (exciton lifetime) is extremely long such as in an order of
several hundred microsecond to millisecond. Therefore, even if the
T.sub.1 energy level of the host compound is higher than that of
the light emitting material, it will be increased the probability
of taking place a reverse energy transfer from the triplet excited
state of the light emitting material to the host compound due to
the long lifetime. As a result, it is difficult to sufficiently
make occur a required reverse intersystem crossing from the triplet
excited state to the singlet excited state of the TADF compound.
Instead, there occurs an unrequired reverse energy transfer to the
host compound as a major route to result in failure to obtain
insufficient emission efficiency.
In order to solve the above-described problem, it is required to
make sharp a shape of an emission spectrum of the TADF compound,
and to decrease the difference between the emission maximum
wavelength and the rise of the emission spectrum. This may be
achieved basically by reducing the change of the molecular
structure of the singlet excited state and the triplet excited
state.
Further, in order to prevent the reverse energy transfer to the
host compound, it is effective to shorten the existing time of the
triplet excited state of the TADF compound (exciton lifetime). In
order to realize this, the possible ways to solve the problem are:
to minimize the molecular structure change between the ground state
and the triplet excited state; and to introduce a suitable
substituent or an element to loosen the forbidden transition.
It will be described a variety of measuring methods concerning a
.pi.-conjugated compound according to the present invention.
[Electron Density Distribution]
From the viewpoint of decreasing .DELTA.E.sub.ST, it is preferable
that a .pi.-conjugated compound according to the present invention
has a HOMO and a LUMO substantially separated with each other in
the molecule. The distribution state of the HOMO and the LUMO may
be obtained from the electron density distribution in the optimized
structure by a molecular orbital calculation.
The structure optimization and the calculation of the electron
density distribution of the .pi.-conjugated compound of the present
invention with a molecular orbital calculation may be done by
employing a software of a molecular orbital calculation using B3LYP
as a functional and 6-31G(d) as a base function for a calculation
method. There is no limitation to the software, the same results
may be obtained with any software.
In the present invention, as a molecular orbital calculation
software, it was used Gaussian 09 made by The US Gaussian Inc.,
(Revision C.01, by M. J. Frisch et al., Gaussian Inc., 2010).
Here, the condition of "a HOMO and a LUMO being substantially
separated" indicates the state in which the center portion of the
HOMO orbital distribution and the center portion of the LUMO
orbital distribution calculated with the above-described molecular
calculation method are separated. More preferably, the HOMO orbital
distribution and the LUMO orbital distribution are substantially
not superimposed.
The separation state of the electron density distribution of the
HOMO and the LUMO may be determined by making calculation of
excited states with a Time-dependent DFT method starting from the
optimized structure calculation using B3LYP as a functional and
6-31G (d) as abase function as described above. The excited state
energy levels of S.sub.1 and T.sub.1 are obtained, and
.DELTA.E.sub.ST is calculated from the scheme of:
.DELTA.E.sub.ST=E(S.sub.1)-E(T.sub.1). The smaller the calculated
.DELTA.E.sub.ST, it indicates that the HOMO and the LUMO are more
separated. In the present invention, an absolute value of
.DELTA.E.sub.ST obtained by the above-described calculation method
is preferably 0.5 eV or less, more preferably it is 0.2 eV or less,
and still more preferably it is 0.1 eV or less.
[Lowest Excited Singlet Energy Level S.sub.1]
In the present invention, the lowest excited singlet energy S1 of
the .pi.-conjugated compound of the present invention may be
determined by a common technique. Specifically, a target compound
is deposited onto a quartz substrate to prepare a sample, and an
absorption spectrum of the sample is measured at ambient
temperature (300 ) (vertical axis: absorbance, horizontal axis:
wavelength). A tangential line is drawn at the rising point of the
absorption spectrum on the longer wavelength side, and the lowest
excited singlet energy is calculated by a specific conversion
expression on the basis of the wavelength at the point of
intersection of the tangential line with the horizontal axis.
When the .pi.-conjugated compound used in the present invention has
a high aggregation property as a molecule itself, it is likely to
cause molecular aggregation, and thus a thin film prepared from the
compound may cause a measurement error due to molecular
aggregation. In the present invention, the lowest excited singlet
energy level is determined from, as an approximation, the peak
wavelength of emission of a solution of the .pi.-conjugated
compound at room temperature (about 25.degree. C.) in consideration
of a relatively small Stokes shift of the .pi.-conjugated compound
and a very small structural change of the compound between the
excited state and the ground state. This determination process may
use a solvent which does not affect the molecular aggregation state
of the .pi.-conjugated compound; for example, a non-polar solvent
having a small solvent effect, such as cyclohexane or toluene.
[Lowest Excited Triplet Energy Level T.sub.1]
The lowest excited triplet energy level (T.sub.1) of the
.pi.-conjugated compound of the present invention is determined on
the basis of the photoluminescent (PL) properties of a solution or
thin film of the compound. For example, a thin film is prepared
from a dilute dispersion of the .pi.-conjugated compound, and the
transient PL properties of the thin film are determined with a
streak camera for separation of a fluorescent component and a
phosphorescent component to determine the absolute value of the
energy difference .DELTA.E.sub.ST therebetween. The lowest excited
triplet energy level may be obtained from the lowest excited
singlet energy level.
For measurement and evaluation, the absolute PL quantum yield was
determined with an absolute PL Quantum yield measuring apparatus
C9920-02 (manufactured by Hamamatsu Photonics K. K.). The emission
lifetime was determined with a streak camera C4334 (manufactured by
Hamamatsu Photonics K.K.) under excitation of the sample with a
laser beam.
<<Constitution Layers of Organic EL Element>>
Representative element constitutions used for an organic EL element
of the present invention are as follows, however, the present
invention is not limited to these.
(1) Anode/light emitting layer/cathode
(2) Anode/light emitting layer/electron transport layer/cathode
(3) Anode/hole transport layer/light emitting layer/cathode
(4) Anode/hole transport layer/light emitting layer/electron
transport layer/cathode
(5) Anode/hole transport layer/light emitting layer/electron
transport layer/electron injection layer/cathode
(6) Anode/hole injection layer/hole transport layer/light emitting
layer/electron transport layer/cathode
(7) Anode/hole injection layer/hole transport layer/(electron
blocking layer/) light emitting layer/(hole blocking layer/)
electron transport layer/electron injection layer/cathode
Among these, the embodiment (7) is preferably used. However, the
present invention is not limited to this.
The light emitting layer of the present invention is composed of
one or a plurality of layers. When a plurality of layers are
employed, it may be placed a non-light emitting intermediate layer
between the light emitting layers.
According to necessity, it may be provided with a hole blocking
layer (it is also called as a hole barrier layer) or an electron
injection layer (it is also called as a cathode buffer layer)
between the light emitting layer and the cathode. Further, it may
be provided with an electron blocking layer (it is also called as
an electron barrier layer) or an hole injection layer (it is also
called as an anode buffer layer) between the light emitting layer
and the anode.
An electron transport layer according to the present invention is a
layer having a function of transporting an electron. An electron
transport layer includes an electron injection layer, and a hole
blocking layer in a broad sense. Further, an electron transport
layer unit may be composed of plural layers.
A hole transport layer according to the present invention is a
layer having a function of transporting a hole. A hole transport
layer includes a hole injection layer, and an electron blocking
layer in a broad sense. Further, a hole transport layer unit may be
composed of plural layers.
In the representative element constitutions as described above, the
layers eliminating an anode and a cathode are also called as
"organic layers".
(Tandem Structure)
An organic EL element of the present invention may be so-called a
tandem structure element in which plural light emitting units each
containing at least one light emitting are laminated.
A representative example of an element constitution having a tandem
structure is as follows.
Anode/first light emitting unit/intermediate layer/second light
emitting unit/intermediate layer/third light emitting
unit/cathode.
Here, the above-described first light emitting unit, second light
emitting unit, and third light emitting unit may be the same or
different. It may be possible that two light emitting units are the
same and the remaining one light emitting unit is different.
The plural light emitting units each may be laminated directly or
they may be laminated through an intermediate layer. Examples of an
intermediate layer are: an intermediate electrode, an intermediate
conductive layer, a charge generating layer, an electron extraction
layer, a connecting layer, and an intermediate insulating layer.
Known composing materials may be used as long as it can form a
layer which has a function of supplying an electron to an adjacent
layer to the anode, and a hole to an adjacent layer to the
cathode.
Examples of a material used in an intermediate layer are:
conductive inorganic compounds such as ITO (indium tin oxide), IZO
(indium zinc oxide), ZnO.sub.2, TiN, ZrN, HfN, TiO.sub.X, VO.sub.X,
CuI, InN, GaN, CuAlO.sub.2, CuGaO.sub.2, SrCu.sub.2O.sub.2,
LaB.sub.6, RuO.sub.2, and Al; a two-layer film such as
Au/Bi.sub.2O.sub.3; a multi-layer film such as
SnO.sub.2/Ag/SnO.sub.2, ZnO/Ag/ZnO,
Bi.sub.2O.sub.3/Au/Bi.sub.2O.sub.3, TiO.sub.2/TiN/TiO.sub.2, and
TiO.sub.2/ZrN/TiO.sub.2; fullerene such as C.sub.60; and a
conductive organic layer such as oligothiophene, metal
phthalocyanine, metal-free phthalocyanine, metal porphyrin, and
metal-free porphyrin. The present invention is not limited to
them.
Examples of a preferable constitution in the light emitting unit
are the constitutions of the above-described (1) to (7) from which
an anode and a cathode are removed. However, the present invention
is not limited to them.
Examples of a tandem type organic EL element are described in: U.S.
Pat. Nos. 6,337,492, 7,420,203,7,473,923,
6,872,472,6,107,734,6,337,492, WO 2005/009087, JP-A 2006-228712,
JP-A 2006-24791, JP-A 2006-49393, JP-A 2006-49394, JP-A 2006-49396,
JP-A 2011-96679, JP-A 2005-340187, JP Patent 4711424, JP Patent
3496681, JP Patent 3884564, JP Patent 4213169, JP-A 2010-192719,
JP-A 2009-076929, JP-A 2008-078414, JP-A 2007-059848, JP-A
2003-272860, JP-A 2003-045676, and WO 2005/094130. The
constitutions of the elements and the composing materials are
described in these documents, however, the present invention is not
limited to them.
Each layer that constitutes an organic EL element of the present
invention will be described in the following.
<<Light Emitting Layer>>
Alight emitting layer according to the present invention is a layer
which provide a place of emitting light via an exciton produce by
recombination of electrons and holes injected from an electrode or
an adjacent layer. The light emitting portion may be either within
the light emitting layer or at an interface between the light
emitting layer and an adjacent layer thereof. The constitution of
the light emitting layer according to the present invention is not
particularly limited as long as it satisfies the requirements of
the present invention.
A total thickness of the light emitting layer is not particularly
limited. However, in view of layer homogeneity, required voltage
during light emission, and stability of the emitted light color
against a drive electric current, the total layer thickness is
preferably adjusted to be in the range of 2 nm to 5 .mu.m, more
preferably, it is in the range of 2 to 500 nm, and still most
preferably, it is in the range of 5 to 200 nm.
Each light emitting layer is preferably adjusted to be in the range
of 2 nm to 1 .mu.m, more preferably, it is in the range of 2 to 200
nm, and still most preferably, it is in the range of 3 to 150
nm.
It is preferable that the light emitting layer of the present
invention incorporates a light emitting dopant (a light emitting
dopant compound, a dopant compound, or simply called as a dopant)
and a host compound (a matrix material, a light emitting host
compound, or simply called as a host). When at least one of the
light emitting layers contains a .pi.-conjugated compound and at
least one of fluorescent compound and a phosphorescent compound,
the emission efficiency is improved. It is preferable. Further,
when at least one of the light emitting layers contains: a
.pi.-conjugated compound; at least one of fluorescent compound and
a phosphorescent compound; and a host compound, the emission
efficiency is improved. It is also preferable.
(1) Light Emitting Dopant
As a light emitting dopant, it is preferable to employ: a
fluorescence emitting dopant (also referred to as a fluorescent
dopant and a fluorescent compound) and a phosphorescence emitting
dopant (also referred to as a phosphorescent dopant and a
phosphorescent emitting material). In the present invention, it is
preferable that at least one light emitting layer contains a
fluorescence emitting dopant. In the present invention, it is
preferable that at least one of the light emitting layers contains
a fluorescent compound (described later) and a .pi.-conjugated
compound served as an assist-dopant.
In the present invention, it is preferable that the light emitting
layer contains a light emitting compound in the range of 0.1 to 50
mass %, more preferably in the range of 1 to 30 mass %.
A concentration of a light emitting compound in a light emitting
layer may be arbitrarily decided based on the specific compound
employed and the required conditions of the device. A concentration
of a light emitting compound may be uniform in a thickness
direction of the light emitting layer, or it may have any
concentration distribution.
It may be used plural light emitting compounds of the present
invention. It may be used a combination of fluorescent compounds
each having a different structure, or a combination of a
fluorescence emitting compound and a phosphorescence emitting
compound. Any required emission color will be obtained by this.
When the light emitting layer contains: a .pi.-conjugated compound
of the present invention having an absolute value of a difference
between a lowest singlet excited level and a lowest triplet level
(.DELTA.E.sub.ST) is 0.5 eV or less; a light emitting compound; and
a host compound, the .pi.-conjugated compound of the present
invention acts as an assist-dopant. Whereas, when the light
emitting layer contains a .pi.-conjugated compound of the present
invention and a light emitting compound without containing a host
compound, the .pi.-conjugated compound of the present invention
acts as a host compound.
The mechanism of appearing the effects is the same for both cases.
The specific feature is that a triplet exciton produced on the
.pi.-conjugated compound of the present invention is converted to a
singlet exciton via a reverse intersystem crossing (RISC).
By this, all energy of the excitons produced on the .pi.-conjugated
compound of the present invention is theoretically transferred to
the light emitting compound. It may be achieved high emission
efficiency.
Consequently, when the light emitting layer contains 3 components
of a .pi.-conjugated compound of the present invention, a light
emitting compound, and a host compound, it is preferable that the
energy levels of S.sub.1 and T.sub.1 of the .pi.-conjugated
compound are lower than the energy levels of S.sub.1 and T.sub.1 of
the host compound, and higher than the energy levels of S.sub.1 and
T.sub.1 of the light emitting compound.
In the same manner, when the light emitting layer contains 2
components of a .pi.-conjugated compound of the present invention
and a light emitting compound, it is preferable that the energy
levels of S.sub.1 and T.sub.1 of the .pi.-conjugated compound are
higher than the energy levels of S.sub.1 and T.sub.1 of the light
emitting compound.
FIG. 7 and FIG. 8 illustrate a schematic diagram of the case in
which the .pi.-conjugated compound of the present invention acts as
an assist-dopant or a host compound. FIG. 7 and FIG. 8 are only an
example, the production process of the triplet exciton on the
.pi.-conjugated compound of the present invention is not limited to
the electric field excitation, the production process includes the
cases of an energy transfer or an electron transfer in the light
emitting layer or from the surrounding interface.
Further, FIG. 7 and FIG. 8 illustrate the diagram using a
fluorescence emitting compound as a light emitting compound,
however, the present invention is not limited to it, and it may be
used a phosphorescence emitting compound, and it may be used both
of a fluorescence emitting compound and a phosphorescence emitting
compound.
When a .pi.-conjugated compound of the present invention is used as
an assist-dopant, it is preferable that the light emitting layer
contains a host compound in an amount of 100 mass % or more with
respect to the .pi.-conjugated compound, and that it contains a
fluorescence emitting compound and/or a phosphorescence emitting
compound in an amount of 0.1 to 50 mass % with respect to the
.pi.-conjugated compound.
When a .pi.-conjugated compound of the present invention is used as
a host compound, it is preferable that the light emitting layer
contains a fluorescence emitting compound and/or a phosphorescence
emitting compound in an amount of 0.1 to 50 mass % with respect to
the .pi.-conjugated compound.
When a .pi.-conjugated compound of the present invention is used as
an assist-dopant or a host compound, it is preferable that an
emission spectrum of the .pi.-conjugated compound of the present
invention and an absorption spectrum of the light emitting compound
are overlapped from the viewpoint of achieving high light emission
efficiency.
Color of light emitted by an organic EL element or a compound of
the present invention is specified as follows. In FIG. 3.16 on page
108 of "Shinpen Shikisai Kagaku Handbook (New Edition Color Science
Handbook)" (edited by The Color Science Association of Japan, Tokyo
Daigaku Shuppan Kai, 1985), values determined via Spectroradiometer
CS-1000 (produced by Konica Minolta, Inc.) are applied to the CIE
chromaticity coordinate, whereby the color is specified.
In the present invention, it is preferable that the organic EL
element of the present invention exhibits white emission by
incorporating one or plural light emitting layers containing plural
emission dopants having different emission colors.
The combination of emission dopants producing white is not
specifically limited. It may be cited, for example, combinations
of: blue and orange; and blue, green and red.
It is preferable that "white" in the organic EL element of the
present invention shows chromaticity in the CIE 1931 Color
Specification System at 1,000 cd/m.sup.2 in the region of
x=0.39.+-.0.09 and y=0.38.+-.0.08, when measurement is done to
2-degree viewing angle front luminance via the aforesaid
method.
(1.1) .pi.-Conjugated Compound
The .pi.-conjugated compound of the present invention has a
direction vector from an atom having a HOMO orbital in the electron
donor portion to an electron cloud of the HOMO orbital, and a
direction vector from an atom having a LUMO orbital in the electron
acceptor portion to an electron cloud of the LUMO orbital, and the
two direction vectors form an angle .theta. in the range of 90 to
180 degrees, and the .pi.-conjugated compound has at least one of a
plurality of the electron donor portions and a plurality of the
electron acceptor portions. It is more preferable that the angle
.theta. is in the range of 135 to 180 degrees.
This .pi.-conjugated compound may be suitably used for a
light-emitting thin film of the present invention described
later.
A donor portion is a porting having an electron donating property.
In the present invention, a HOMO designates a .pi.-orbital or an
n-orbital localized in a donor portion. Here, "portion" in the
donor portion indicates a substituent or an atomic group.
Examples of a donor portion are: arylamine derivatives, carbazole,
phenoxazine, 9,10-dihydroacrydine, and phenothiazine.
An acceptor portion is an electron withdrawing portion that is
electron deficient. In the present invention, a LUMO designates a
.pi.*-orbital or a .sigma.*-orbital localized in an acceptor
portion. Here, "portion" in the acceptor portion indicates a
substituent or an atomic group.
Examples of an acceptor portion are: a benzene ring substituted
with a cyano group, a triazine ring, a pyrimidine ring, a boron
atom, and a sulfonyl group.
It will be described a direction vector from an atom having a HOMO
orbital in the electron donor portion to an electron cloud of the
HOMO orbital, and a direction vector from an atom having a LUMO
orbital in the electron acceptor portion to an electron cloud of
the LUMO orbital. The electron cloud of the HOMO orbital in the
electron donor portion indicates an electron cloud of a
.pi.-orbital or an n-orbital in the donor portion. The electron
cloud of the LUMO orbital in the electron acceptor portion
indicates an electron cloud of a .pi.*-orbital or a
.sigma.*-orbital in the acceptor portion. The directions of
.pi.-orbital, n-orbital, .pi.*-orbital and .sigma.*-orbital
extended from the atom are known by the molecular orbital method.
For example, the .pi.-orbital composed of sp2 hybrid orbitals has a
direction of a 2 pz orbital as a direction vector of the present
invention.
The angle .theta. according to the present invention is an angle
formed with a direction vector from an atom having a HOMO orbital
in the electron donor portion to an electron cloud of the HOMO
orbital, and a direction vector from an atom having a LUMO orbital
in the electron acceptor portion to an electron cloud of the LUMO
orbital.
The angle .theta. according to the present invention may be
calculated using a molecular orbital calculation software of
Gaussian 09 made by The US Gaussian Inc., (Revision C.01, by M. J.
Frisch et al., Gaussian Inc., 2010) with B3LYP as a functional and
6-31G(d) as a base function for a calculation method. The
calculation software and the calculation method are not limited, it
may be obtained the same results by using any method.
The .pi.-conjugated compound of the present invention has an
electron donor portion and an electron acceptor portion in the
molecule. It is preferable that the .pi.-conjugated compound has
one electron acceptor portion that is bonded to two or more
electron donor portions through a linking group, or it has one
electron donor portion that is bonded to two or more electron
acceptor portions through a linking group.
Further, it is also preferable that the .pi.-conjugated compound of
the present invention has one electron acceptor portion that is
directly bonded to two or more electron donor portions, or it has
one electron donor portion that is directly bonded to two or more
electron acceptor portions.
Specific examples of a preferable .pi.-conjugated compound of the
present invention are a .pi.-conjugated compound represented by any
one of Formulas (1) to (8). At least one of the light emitting
layers according to the present invention preferably contains at
least one of these .pi.-conjugated compounds.
##STR00002##
In Formulas, X.sup.1 to X.sup.8 and Y.sup.1 to Y.sup.20 each
respectively represent the electron donor portion or the electron
acceptor portion; when X.sup.1 to X.sup.8 each respectively
represent the electron donor portion, Y.sup.1 to Y.sup.20 each
respectively represent the electron acceptor portion; when to
X.sup.8 each respectively represent the electron acceptor portion,
Y.sup.1 to Y.sup.20 each respectively represent the electron donor
portion; L.sup.1 to L.sup.10 represent a linking group, L.sup.1 to
L.sup.10 each respectively represent an aryl group which may have a
substituent or a heteroaryl group which may have a substituent,
L.sup.1 binds X.sup.1 and Y.sup.1 through adjacent carbon atoms,
L.sup.2 binds X.sup.1 and Y.sup.2 through adjacent carbon atoms,
L.sup.3 binds X.sup.1 and Y.sup.3 through adjacent carbon atoms,
L.sup.4 binds X.sup.1 and Y.sup.4 through adjacent carbon atoms,
L.sup.5 binds X.sup.2 and Y.sup.5 through adjacent carbon atoms,
L.sup.6 binds X.sup.2 and Y.sup.6 through adjacent carbon atoms,
L.sup.7 binds X.sup.2 and Y.sup.7 through adjacent carbon atoms,
L.sup.8 binds X.sup.3 and Y.sup.8 through adjacent carbon atoms,
L.sup.9 binds X.sup.3 and Y.sup.9 through adjacent carbon atoms,
and L.sup.10 binds X.sup.7 and Y1.sup.9 through adjacent carbon
atoms.
Further, it is preferable that the electron donor portion and the
electron acceptor portion represented by X.sup.1 to X.sup.8 and
Y.sup.1 to Y.sup.20 in Formulas (1) to (8) each respectively are
one selected from the group consisting of an aryl group which may
have a substituent, a heteroaryl group which may have a
substituent, an alkyl group which may have a substituent, a
carbonyl group which may have a substituent, a nitrogen atom which
may have a substituent, a sulfur atom which may have a substituent,
a boron atom which may have a substituent, a phosphor atom which
may have a substituent, an oxygen atom which may have a
substituent, and a silicon atom which may have a substituent.
Further, it is preferable that the electron acceptor portions
represented by X.sup.1 to X.sup.8 and Y.sup.1 to Y.sup.20 in
Formulas (1) to (8) each respectively are one selected from the
group consisting of: an aryl ring having 6 to 20 carbon atoms which
may by partially substituted with an alkyl group, an aryl ring
having 6 to 20 carbon atoms which may by partially substituted with
an alkoxy group, a carbazole ring which may have a substituent, an
indoloindole ring which may have a substituent, a
9,10-dihydroacrydine ring which may have a substituent, a
phenoxazine ring which may have a substituent, a phenothiazine ring
which may have a substituent, a 5,10-dihydrophenazine ring which
may have a substituent, a dibenzothiophene ring which may have a
substituent, an amino group which may have a substituent, and a
thio group which may have a substituent.
Further, it is preferable that the electron acceptor portions
represented by X.sup.1 to X.sup.8 and Y.sup.1 to Y.sup.20 in
Formulas (1) to (8) each respectively are one selected from the
group consisting of: an aryl ring having 6 to 20 carbon atoms which
may be partially substituted with a cyano group, an aryl ring
having 6 to 20 carbon atoms which may be partially substituted with
a fluoroalkyl group, an aryl ring having 6 to 20 carbon atoms which
may be partially or wholly substituted with a fluorine atom, a
nitrogen atom-containing aromatic ring having 5 to 13 carbon atoms
which may have a substituent, a dibenzoborol ring which may have a
substituent, a dibenzothiphene oxide ring which may have a
substituent, a dibenzothiphene dioxide ring which may have a
substituent, a sulfinyl group which may have a substituent, a
sulfonyl group which may have a substituent, a boryl group which
may have a substituent, a phosphine oxide group which may have a
substituent, a silyl group which may have a substituent, a pyridine
ring, a pyrdazine ring, a pyrimidine ring, a pyrazine ring, a
triazine ring, a quinoline ring, an isoquinoline ring, a
quinazoline ring, a cinnoline ring, a quinoxaline ring, a
phthalazine ring, a pteridine ring, an acridine ring, a
phenanthridine ring, and a phenanthroline ring. It is particularly
preferable that they are one selected from the group consisting of:
a pyridine ring, a pyridazine ring, a pyrimidine ring, a pyrazine
ring, a triazine ring, a quinoline ring, an isoquinoline ring, a
phenanthridine ring, and a phenanthroline ring.
Specific examples of the aryl group having 6 to 20 carbon atoms
are: a benzene ring, an indene ring, a naphthalene ring, an azulene
ring, a fluorene ring, a phenanthrene ring, an anthracene ring, an
acenaphthylene ring, a biphenylene ring, a chrysene ring, a
naphthacene ring, a pyrene ring, a pentalene ring, an aceanthrylene
ring, a heptalene ring, a triphenylene ring, an as-indacene ring, a
chrysene ring, an s-indacene ring, a pleiadene ring, a phenalene
ring, a fluoranthene ring, a perylene ring, and an
acephenanthrylene ring. More preferable examples are: a benzene
ring, a naphthalene ring, a fluorene ring, a phenanthrene ring, an
anthracene ring, a biphenylene ring, a chrysene ring, a pyrene
ring, a triphenylene ring, a chrysene ring, a fluoranthene ring,
and a perylene ring. Particularly preferable examples are: a
benzene ring, a naphthalene ring, a phenanthrene ring, and a pyrene
ring.
The above-describe alkyl group may be straight, branched or cyclic.
Examples thereof are: a straight, branched or cyclic alkyl group
having 1 to 20 carbon atoms. Specific examples are: a methyl group,
an ethyl group, an n-propyl group, an isopropyl group, an n-butyl
group, an s-butyl group, a t-butyl group, an n-pentyl group, a
neopentyl group, an n-hexyl group, a cyclohexyl group, a
2-ethylhexyl group, an n-heptyl group, an n-octyl group, a
2-hexyloctyl group, an n-nonyl group, an n-decyl group, an
n-undecyl group, an n-dodecyl group, an n-tridecyl group, an
n-tetradecyl group, an n-pentadecyl group, an n-hexadecyl group, an
n-heptadecyl group, an n-octadecyl group, an n-nonadecyl group, and
an n-icosyl group. More preferable examples are: a methyl group, an
ethyl group, an isopropyl group, a t-butyl group, a cyclohexyl
group, a 2-ethylhexyl group, and 2-hexyloctyl group.
The above-describe alkoxy group may be straight, branched or
cyclic. Examples thereof are: a straight, branched or cyclic alkoxy
group having 1 to 20 carbon atoms. Specific examples are: a methoxy
group, an ethoxy group, an n-propoxy group, an isopropoxy group, an
n-butoxy group, an isobutoxy group, a t-butoxy group, an
n-pentyloxy group, a neopentyloxy group, an n-hexyloxy group, a
cyclohexyloxy group, an n-heptyloxy group, an n-octyloxy group, a
2-ethylhexyloxy group, a nonyloxy group, a decyloxy group, a 3,
7-dimethyloctyloxy group, an n-undecyloxy group, an n-dodecyloxy
group, an n-tridecyloxy group, an n-tetradecyloxy group, a
2-n-hexyl-n-octyloxy group, an n-pentadecyloxy group, an
n-hexadecyloxy group, an n-heptadecyloxy group, an n-octadecyloxy
group, an n-nonadecyloxy group, and an n-icosyloxy group. More
preferable examples are: a methoxy group, an ethoxy group, an
isopropoxy group, a t-butoxy group, a cyclohexyloxy group, a
2-ethylhexyloxy group, and a 2-hexyloctyloxy group.
The above-describe linking group (including L.sup.1 to L.sup.10 in
Formulas (1) to (3) and (7)) is not limited in particular as long
as it does not hinder the effect of the present invention.
Preferable examples thereof are: a benzene ring, a naphthalene
ring, a thiophene ring, a furan ring, a benzofuran ring, a
benzothiophene ring, and a thienothiophene ring. It is particularly
preferable that L.sup.1 to L.sup.10 in Formulas (1) to (3) and (7)
each are a benzene ring.
As a .pi.-conjugated compound represented by any one of Formulas
(1) to (8), it may be cited the following compounds. However, the
present invention is not limited to them.
In addition, the following compounds all have the above-described
angle .theta. in the range of 90 to 180 degrees.
##STR00003## ##STR00004## ##STR00005## ##STR00006## ##STR00007##
##STR00008## ##STR00009## ##STR00010## ##STR00011## ##STR00012##
##STR00013## ##STR00014## ##STR00015## ##STR00016## ##STR00017##
##STR00018## ##STR00019## ##STR00020## ##STR00021## ##STR00022##
##STR00023## ##STR00024##
By using these compounds, it is possible to achieve a structure in
which an electron transition will be easily taken place from a
donor portion to an acceptor portion. In addition, among these
compounds, the materials having .DELTA.E.sub.ST (an absolute value)
in the range of 0.5 eV or less may exhibit a TADF property.
Further, since these compounds have a bipolar property and they may
be compatible with a variety of energy levels, they may be used as
an emission host, and they may be suitably used as a hole transport
compound or an electron transport compound. Consequently, the use
of these compounds is not limited to a light emission layer, they
may be used in the hole injection layer, a hole transport layer, an
electron blocking layer, a hole blocking layer, an electron
transport layer, an electron injection layer, or an intermediate
layer.
<Synthetic Method>
The above-described .pi.-conjugated compound may be synthesized
with the methods described in Non-patent document 2: Journal of
Organometallic Chemistry, 2003, 680, 218-222 and WO 2011/8560 or by
referring to the methods described in the references of these
documents.
(1.2) Fluorescence Emitting Dopant
As a fluorescent dopant, it may be used a .pi.-conjugated compound
of the preset invention. Otherwise, it may be suitably selected
from the known fluorescent dopants and delayed fluorescent dopants
used in a light emitting layer of an organic EL element.
As specific known fluorescence emitting dopants usable in the
present invention, listed are compounds such as: an anthracene
derivative, a pyrene derivative, a chrysene derivative, a
fluoranthene derivative, a perylene derivative, a fluorene
derivative, an arylacetylene derivative, a styrylarylene
derivative, a styrylamine derivative, an arylamine derivative, a
boron complex, a coumarin derivative, a pyran derivative, a cyanine
derivative, a croconium derivative, a squarium derivative, an
oxobenzanthracene derivative, a fluorescein derivative, a rhodamine
derivative, a pyrylium derivative, a perylene derivative, a
polythiophene derivative, and a rare earth complex compound.
In addition, it has been developed a light emitting dopant
utilizing delayed fluorescence. It may be used a light emitting
dopant utilizing this type of fluorescence. Specific examples of
utilizing delayed fluorescence are compounds described in: WO
2011/156793, JP-A 2011-213643, and JP-A 2010-93181. However, the
present invention is not limited to them.
(1.3) Phosphorescence Emitting Dopant
A phosphorescence emitting dopant according to the present
invention will be described.
The phosphorescence emitting dopant according to the present
invention is a compound which is observed emission from an excited
triplet state thereof. Specifically, it is a compound which emits
phosphorescence at a room temperature (25.degree. C.) and exhibits
a phosphorescence quantum yield of at least 0.01 at 25.degree. C.
The phosphorescence quantum yield is preferably at least 0.1.
The phosphorescence quantum yield will be determined via a method
described in page 398 of Bunko II of Dai 4 Han Jikken Kagaku Koza 7
(Spectroscopy II of 4th Edition Lecture of Experimental Chemistry
7) (1992, published by Maruzen Co. Ltd.). The phosphorescence
quantum yield in a solution will be determined using appropriate
solvents. However, it is only necessary for the phosphorescent
dopant of the present invention to exhibit the above
phosphorescence quantum yield (0.01 or more) using any of the
appropriate solvents.
A phosphorescence dopant may be suitably selected and employed from
the known materials used for a light emitting layer for an organic
EL element.
Examples of a known phosphorescence dopant are compound described
in the following publications.
Nature 395, 151 (1998), Appl. Phys. Lett. 78, 1622 (2001), Adv.
Mater. 19, 739 (2007), Chem. Mater. 17, 3532 (2005), Adv. Mater.
17, 1059 (2005), WO 2009/100991, WO 2008/101842, WO 2003/040257, US
2006/835469, US 2006/0202194, US 2007/0087321, US 2005/0244673,
Inorg. Chem. 40, 1704 (2001), Chem. Mater. 16, 2480 (2004), Adv.
Mater. 16, 2003 (2004), Angew. Chem. Int. Ed. 2006, 45, 7800, Appl.
Phys. Lett. 86, 153505 (2005), Chem. Lett. 34, 592 (2005), Chem.
Commun. 2906 (2005), Inorg. Chem. 42, 1248 (2003), WO 2009/050290,
WO 2002/015645, WO 2009/000673, US 2002/0034656, U.S. Pat. No.
7,332,232, US 2009/0108737, US 2009/0039776, U.S. Pat. Nos.
6,921,915, 6,687,266, US 2007/0190359, US 2006/0008670, US
2009/0165846, US 2008/0015355, U.S. Pat. Nos. 7,250,226, 7,396,598,
US 2006/0263635, US 2003/0138657, US 2003/0152802, U.S. Pat. No.
7,090,928, Angew. Chem. Int. Ed. 47, 1 (2008), Chem. Mater. 18,
5119 (2006), Inorg. Chem. 46, 4308 (2007), Organometallics 23, 3745
(2004), Appl. Phys. Lett. 74, 1361 (1999), WO 2002/002714, WO
2006/009024, WO 2006/056418, WO 2005/019373, WO 2005/123873, WO
2005/123873, WO 2007/004380, WO 2006/082742, US 2006/0251923, US
2005/0260441, U.S. Pat. Nos. 7,393,599, 7,534,505, 7,445,855, US
2007/0190359, US 2008/0297033, U.S. Pat. No. 7,338,722, US
2002/0134984, and U.S. Pat. No. 7,279,704, US 2006/098120, US
2006/103874, WO 2005/076380, WO 2010/032663, WO 2008/140115, WO
2007/052431, WO 2011/134013, WO 2011/157339, WO 2010/086089, WO
2009/113646, WO 2012/020327, WO 2011/051404, WO 2011/004639, WO
2011/073149, JP-A 2012-069737, JP Application No. 2011-181303, JP-A
2009-114086, JP-A 2003-81988, JP-A 2002-302671 and JP-A
2002-363552.
Among them, preferable phosphorescence emitting dopants are organic
metal complexes containing Ir as a center metal. More preferable
are complexes containing at least one coordination mode selected
from a metal-carbon bond, a metal-nitrogen bond, a metal-oxygen
bond and a metal-sulfur bond.
(2) Host Compound
A host compound according to the present invention is a compound
which mainly plays a role of injecting or transporting a charge in
a light emitting layer. In an organic EL element, an emission from
the host compound itself is substantially not observed.
Among the compounds incorporated in the light emitting layer, a
mass ratio of the host compound in the aforesaid layer is
preferably at least 20%.
Host compounds may be used singly or may be used in combination of
two or more compounds. By using plural host compounds, it is
possible to adjust transfer of charge, thereby it is possible to
achieve high efficiency of an organic EL element.
In the following, preferable host compounds used in the present
invention will be described.
A host compound may be a .pi.-conjugated compound used in the
present invention as described above. However it is not
specifically limited to that. From the viewpoint of a reverse
energy transfer, it is preferable that the host compound has a
larger excited energy level than an excited singlet energy level of
the dopant compound. It is more preferable that the host compound
has a larger excited triplet energy level than an excited triplet
energy level of the dopant.
A host compound bears the function of transfer of the carrier and
generation of an exciton in the light emitting layer. Therefore, it
is preferable that the host compound will exist in all of the
active species of a cation radical state, an anion radial state and
an excited state, and that it will not make chemical reactions such
as decomposition and addition. Further, it is preferable that the
host molecule will not move in the layer with an Angstrom level
when an electric current is applied.
In particular, when the jointly used light emitting dopant exhibits
TADF emission, since the lifetime of the triplet excited state of
the TADF material is long, it is required an appropriate design of
a molecular structure to prevent the host compound from having a
lower T.sub.1 level such as: the host compound has a high T.sub.1
energy; the host compounds will not form a low T.sub.1 state when
aggregated each other; the TADF material and the host compound will
not form an exciplex; and the host compound will not form an
electromer by applying an electric field.
In order to satisfy the above-described requirements, it is
required that: the host compound itself has a high hopping
mobility; the host compound has high hole hopping mobility; and the
host compound has small structural change when it becomes a triplet
excited state. As a representative host compound satisfying these
requirements, preferable compounds are: a compound having a high
T.sub.1 energy such as a carbazole structure, an azacarbazole
structure, a dibenzofuran structure, a dibenzothiophene structure
and an azadibenzofuran structure. In particular, when the light
emitting layer contains a carbazole derivative, it will promote
suitable carrier hopping in the light emitting layer and suitable
dispersion of the emitting material. Thereby it may be obtained the
effect of improved emitting property and improved stability of the
thin layer. It is a preferable embodiment.
A host compound has a hole transporting ability or an electron
transporting ability, as well as preventing elongation of an
emission wavelength. In addition, from the viewpoint of stably
driving an organic EL element at high temperature, it is preferable
that a host compound has a high glass transition temperature (T) of
90.degree. C. or more, more preferably, has a Tg of 120.degree. C.
or more.
Here, a glass transition temperature (Tg) is a value obtained using
DSC (Differential Scanning Colorimetry) based on the method in
conformity to JIS-K-7121-2012.
A host compound suitably used in the present invention is a
.pi.-conjugated compound according to the present invention as
described above. The reason of this is that the .pi.-conjugated
compound of the present invention has a condensed ring structure
and the .pi.-electron cloud is extended. As a result, the compound
has high carrier transport ability and a high glass transition
temperature (Tg). Further, the .pi.-conjugated compound of the
present invention has a high triplet energy (T.sub.1), and it is
appropriately used for an emission of short wavelength (namely,
having large T.sub.1 and S.sub.1).
As specific examples of a known host compound used in an organic EL
element of the present invention, the compounds described in the
following Documents are cited. However, the present invention is
not to them.
Japanese patent application publication (JP-A) Nos. 2001-257076,
2002-308855, 2001-313179, 2002-319491, 2001-357977, 2002-334786,
2002-8860, 2002-334787, 2002-15871, 2002-334788, 2002-43056,
2002-334789, 2002-75645, 2002-338579, 2002-105445, 2002-343568,
2002-141173, 2002-352957, 2002-203683, 2002-363227, 2002-231453,
2003-3165, 2002-234888, 2003-27048, 2002-255934, 2002-260861,
2002-280183, 2002-299060, 2002-302516, 2002-305083, 2002-305084 and
2002-308837; US Patent Application Publication (US) Nos.
2003/0175553, 2006/0280965, 2005/0112407, 2009/0017330,
2009/0030202, 2005/0238919; WO 2001/039234, WO2009/021126, WO
2008/056746, WO 2004/093 207, WO 2005/089025, WO 2007/063796,
WO2007/063754, WO2004/107822, WO2005/030900, WO2006/114966, WO
2009/086028, WO 2009/003898, WO 2012/023947, JP-A 2008-074939, JP-A
2007-254297, EP 2034538, WO 2011/055933, and WO 2012/035853. The
specific host compounds which may be used in the present invention
are: Compounds H-1 to H-231 in paragraphs [0255] to [0293] of JP-A
No. 2015-38941, or H-232 to H-236 as described in the following.
However, the host compounds in the present invention are not
limited to them.
##STR00025##
A preferable host compound used for the present invention may be a
low molecular weight compound which has a molecular weight enabling
to be purified with sublimation, or it may be a polymer having a
repeating unit.
The low molecular weight compound has an advantage of obtaining a
highly purified material since it is possible to purify with
sublimation. The molecular weight thereof is not specifically
limited as long as it is possible to purify with sublimation. A
preferable molecular weight is 3,000 or less, and a more preferable
molecular weight is 2,000 or less.
A polymer or an oligomer having a repeating unit has an advantage
of easily forming a film with a wet process. In addition, since a
polymer has generally a high Tg, the polymer is preferable from the
viewpoint of heat resistivity.
<<Electron Transport Layer>>
An electron transport layer of the present invention is composed of
a material having a function of transferring an electron. It is
only required to have a function of transporting an injected
electron from a cathode to a light emitting layer.
A total layer thickness of the electron transport layer is not
specifically limited, however, it is generally in the range of 2 nm
to 5 .mu.m, and preferably, it is in the range of 2 to 500 nm, and
more preferably, it is in the range of 5 to 200 nm.
In an organic EL element of the present invention, it is known that
there occurs interference between the light directly taken from the
light emitting layer and the light reflected at the electrode
located at the opposite side of the electrode from which the light
is taken out at the moment of taking out the light which is
produced in the light emitting layer. When the light is reflected
at the cathode, it is possible to use effectively this interference
effect by suitably adjusting the total thickness of the electron
transport layer in the range of several nm to several .mu.m.
On the other hand, the voltage will be increased when the layer
thickness of the electron transport layer is made thick. Therefore,
especially when the layer thickness is large, it is preferable that
the electron mobility in the electron transport layer is
1.times.10.sup.-5 cm.sup.2/Vs or more.
As a material used for an electron transport layer (hereafter, it
is called as an electron transport material), it is only required
to have either a property of ejection or transport of electrons, or
a barrier to holes. Any of the conventionally known compounds may
be selected and they may be employed.
Cited examples thereof include: a nitrogen-containing aromatic
heterocyclic derivative (a carbazole derivative, an azacarbazole
derivative (a compound in which one or more carbon atoms
constituting the carbazole ring are substitute with nitrogen
atoms), a pyridine derivative, a pyrimidine derivative, a pyrazine
derivative, a pyridazine derivative, a triazine derivative, a
quinoline derivative, a quinoxaline derivative, a phenanthroline
derivative, an azatriphenylene derivative, an oxazole derivative, a
thiazole derivative, an oxadiazole derivative, a thiadiazole
derivative, a triazole derivative, a benzimidazole derivative, a
benzoxazole derivative, and a benzothiazole derivative); a
dibenzofuran derivative, a dibenzothiophene derivative, a silole
derivative; and an aromatic hydrocarbon ring derivative (a
naphthalene derivative, an anthracene derivative and a triphenylene
derivative).
Further, metal complexes having a ligand of a 8-quinolinol
structure or dibnenzoquinolinol structure such as
tris(8-quinolinol)aluminum (Alq.sub.3), tris(5,
7-dichloro-8-quinolinol)aluminum, tris(5,
7-dibromo-8-quinolinol)aluminum,
tris(2-methyl-8-quinolinol)aluminum,
tris(5-methyl-8-quinolinol)aluminum and bis(8-quinolinol)zinc
(Znq); and metal complexes in which a central metal of the
aforesaid metal complexes is substituted by In, Mg, Cu, Ca, Sn, Ga
or Pb, may be also utilized as an electron transport material.
Further, a metal-free or metal phthalocyanine, or a compound whose
terminal is substituted by an alkyl group or a sulfonic acid group,
may be preferably utilized as an electron transport material. A
distyryl pyrazine derivative, which is exemplified as a material
for a light emitting layer, may be used as an electron transport
material. Further, in the same manner as used for a hole injection
layer and a hole transport layer, an inorganic semiconductor such
as an n-type Si and an n-type SiC may be also utilized as an
electron transport material.
It may be used a polymer material introduced these compounds in the
polymer side-chain or a polymer material having any one of these
substance in a polymer main chain.
In an electron transport layer according to the present invention,
it is possible to employ an electron transport layer of a higher n
property (electron rich) which is doped with impurities as a guest
material. As examples of a dope material, listed are those
described in each of JP-A Nos. 4-297076, 10-270172, 2000-196140,
2001-102175, as well as in J. Appl. Phys., 95, 5773 (2004).
Although the present invention is not limited thereto, preferable
examples of a known electron transport material used in an organic
EL element of the present invention are compounds described in the
following publications.
U.S. Pat. Nos. 6,528,187, 7,230,107, US 2005/0025993, US
2004/0036077, US 2009/0115316, US 2009/0101870, US 2009/0179554,
WO2003/060956, WO2008/132085, Appl. Phys. Lett. 75, 4 (1999), Appl.
Phys. Lett. 79, 449 (2001), Appl. Phys. Lett. 81, 162 (2002), Appl.
Phys. Lett. 81, 162 (2002), Appl. Phys. Lett. 79, 156 (2001), U.S.
Pat. No. 7,964,293, US 2009/030202, WO 2004/080975, WO 2004/063159,
WO 2005/085387, WO 2006/067931, WO2007/086552, WO2008/114690,
WO2009/069442, WO2009/066779, WO2009/054253, WO2011/086935,
WO2010/150593, WO2010/047707, EP 2311826, JP-A 2010-251675, JP-A
2009-209133, JP-A 2009-124114, JP-A 2008-277810, JP-A 2006-156445,
JP-A 2005-340122, JP-A 2003-45662, JP-A 2003-31367, JP-A
2003-282270, and WO 2012/115034.
As a preferable electron transport material, it may be cited an
aromatic heterocyclic ring compound containing at least one
nitrogen atom. Examples thereof are: a pyridine derivative, a
pyrimidine derivative, a pyrazine derivative, a triazine
derivative, a dibenzofuran derivative, a dibenzothiophene
derivative, a carbazole derivative, an azacarbazole derivative, and
a benzimidazole derivative. An electron transport material may be
used singly, or may be used in combination of plural kinds of
compounds.
<<Hole Blocking Layer>>
A hole blocking layer is a layer provided with a function of an
electron transport layer in a broad meaning. Preferably, it
contains a material having a function of transporting an electron,
and having very small ability of transporting a hole. It will
improve the recombination probability of an electron and a hole by
blocking a hole while transporting an electron.
Further, a composition of an electron transport layer described
above may be appropriately utilized as a hole blocking layer of the
present invention when needed.
A hole blocking layer placed in an organic EL element of the
present invention is preferably arranged at a location in the light
emitting layer adjacent to the cathode side.
A thickness of a hole blocking layer according to the present
invention is preferably in the range of 3 to 100 nm, and more
preferably, in the range of 5 to 30 nm.
With respect to a material used for a hole blocking layer, the
material used in the aforesaid electron transport layer is suitably
used, and further, the material used as the aforesaid host compound
is also suitably used for a hole blocking layer.
<<Electron Injection Layer>>
An electron injection layer (it is also called as "a cathode buffer
layer") according to the present invention is a layer which is
arranged between a cathode and a light emitting layer to decrease
an operating voltage and to improve an emission luminance. An
example of an electron injection layer is detailed in volume 2,
chapter 2 "Electrode materials" (pp. 123-166) of "Organic EL
Elements and Industrialization Front thereof (Nov. 30, 1998,
published by N.T.S. Co. Ltd.)".
In the present invention, an electron injection layer is provided
according to necessity, and as described above, it is placed
between a cathode and a light emitting layer, or between a cathode
and an electron transport layer.
An electron injection layer is preferably a very thin layer. The
layer thickness thereof is preferably in the range of 0.1 to 5 nm
depending on the materials used.
An election injection layer is detailed in JP-A Nos. 6-325871,
9-17574, and 10-74586. Examples of a material preferably used in an
election injection layer include: a metal such as strontium and
aluminum; an alkaline metal compound such as lithium fluoride,
sodium fluoride, or potassium fluoride; an alkaline earth metal
compound such as magnesium fluoride; a metal oxide such as aluminum
oxide; and a metal complex such as lithium 8-hydroxyquinolate
(Liq). It is possible to use the aforesaid electron transport
materials.
The above-described materials may be used singly or plural kinds
may be used together in an election injection layer.
<<Hole Transport Layer>>
In the present invention, a hole transport layer contains a
material having a function of transporting a hole. A hole transport
layer is only required to have a function of transporting a hole
injected from an anode to a light emitting layer.
The total layer thickness of a hole transport layer of the present
invention is not specifically limited, however, it is generally in
the range of 0.5 nm to 5 .mu.m, preferably in the range of 2 to 500
nm, and more preferably in the range of 5 to 200 nm.
A material used in a hole transport layer (hereafter, it is called
as a hole transport material) is only required to have any one of
properties of injecting and transporting a hole, and a barrier
property to an electron. A hole transport material may be suitably
selected from the conventionally known compounds.
Examples of a hole transport material include: a porphyrin
derivative, a phthalocyanine derivative, an oxazole derivative, an
oxadiazole derivative, a triazole derivative, an imidazole
derivative, a pyrazoline derivative, a pyrazolone derivative, a
phenylenediamine derivative, a hydrazone derivative, a stilbene
derivative, a polyarylalkane derivative, a triarylamine derivative,
a carbazole derivative, an indolocarbazole derivative, an isoindole
derivative, an acene derivative of anthracene or naphthalene, a
fluorene derivative, a fluorenone derivative, polyvinyl carbazole,
a polymer or an oligomer containing an aromatic amine in a side
chain or a main chain, polysilane, and a conductive polymer or an
oligomer (e.g., PEDOT:PSS, an aniline type copolymer, polyaniline
and polythiophene).
Examples of a triarylamine derivative include: a benzidine type
represented by .alpha.-NPD
(4,4'-bis[N-(1-naphthyl)-N-phenyamino]biphenyl), a star burst type
represented by MTDATA
(4,4',4''-tris(N-(3-methylphenyl)-N-phenylamino) triphenylamine), a
compound having fluorenone or anthracene in a triarylamine bonding
core.
A hexaazatriphenylene derivative described in JP-A Nos. 2003-519432
and 2006-135145 may be also used as a hole transport material.
In addition, it is possible to employ an electron transport layer
of a higher p property which is doped with impurities. As its
example, listed are those described in each of JP-A Nos. 4-297076,
2000-196140, and 2001-102175, as well as in J. Appl. Phys., 95,
5773 (2004).
Further, it is possible to employ so-called p-type hole transport
materials, and inorganic compounds such as p-type Si and p-type
SiC, as described in JP-A No. 11-251067, and J. Huang et al.
reference (Applied Physics Letters 80 (2002), p. 139). Moreover, an
orthometal compounds having Ir or Pt as a center metal represented
by Ir(ppy).sub.3 are also preferably used.
Although the above-described compounds may be used as a hole
transport material, preferably used are: a triarylamine derivative,
a carbazole derivative, an indolocarbazole derivative, an
azatriphenylene derivative, an organic metal complex, a polymer or
an oligomer incorporated an aromatic amine in a main chain or in a
side chain.
Specific examples of a known hole transport material used in an
organic EL element of the present invention are compounds in the
aforesaid publications and in the following publications. However,
the present invention is not limited to them.
Examples of a publication are: Appl. Phys. Lett. 69, 2160(1996), J.
Lumin. 72-74, 985(1997), Appl. Phys. Lett. 78, 673(2001), Appl.
Phys. Lett. 90, 183503(2007), Appl. Phys. Lett. 51, 913(1987),
Synth. Met. 87, 171(1997), Synth. Met. 91, 209(1997), Synth. Met.
111, 421(2000), SID Symposium Digest, 37, 923(2006), J. Mater.
Chem. 3, 319(1993), Adv. Mater. 6, 677(1994), Chem. Mater. 15,
3148(2003), US 2003/0162053, US 2002/0158242, US 2006/0240279, US
2008/0220265, U.S. Pat. No. 5,061,569, WO 2007/002683, WO
2009/018009, EP 650955, US 2008/0124572, US 2007/0278938, US
2008/0106190, US 2008/0018221, WO 2012/115034, JP-A 2003-519432,
JP-A 2006-135145, and U.S. patent application Ser. No.
13/585,981.
A hole transport material may be used singly or may be used in
combination of plural kinds of compounds.
<<Electron Blocking Layer>>
An electron blocking layer is a layer provided with a function of a
hole transport layer in a broad meaning. Preferably, it contains a
material having a function of transporting a hole, and having very
small ability of transporting an electron. It will improve the
recombination probability of an electron and a hole by blocking an
electron while transporting a hole. Further, a composition of a
hole transport layer described above may be appropriately utilized
as an electron blocking layer of an organic EL element of the
present invention when needed.
An electron blocking layer placed in an organic EL element of the
present invention is preferably arranged at a location in the light
emitting layer adjacent to the anode side.
A thickness of an electron blocking layer is preferably in the
range of 3 to 100 nm, and more preferably, in the range of 5 to 30
nm.
With respect to a material used for an electron blocking layer, the
material used in the aforesaid hole transport layer is suitably
used, and further, the material used as the aforesaid host compound
is also suitably used for an electron blocking layer.
<<Hole Injection Layer>>
A hole injection layer (it is also called as "an anode buffer
layer") is a layer which is arranged between an electrode and a
light emitting layer to decrease an operating voltage and to
improve an emission luminance. An example of a hole injection layer
is detailed in volume 2, chapter 2 "Electrode materials" (pp.
123-166) of "Organic EL Elements and Industrialization Front
thereof (Nov. 30, 1998, published by N.T.S. Co. Ltd.)".
A hole injection layer is provided according to necessity, and as
described above, it is placed between an anode and a light emitting
layer, or between an anode and a hole transport layer.
A hole injection layer is also detailed in JP-A Nos. 9-45479,
9-260062 and 8-288069. Materials used in the hole injection layer
are the same materials used in the aforesaid hole transport
layer.
Among them, preferable materials are: a phthalocyanine derivative
represented by copper phthalocyanine; a hexaazatriphenylene
derivative described in JP-A Nos. 2003-519432 and 2006-135145; a
metal oxide represented by vanadium oxide; a conductive polymer
such as amorphous carbon, polyaniline (or called as emeraldine) and
polythiophene; an orthometalated complex represented by
tris(2-phenylpyridine) iridium complex; and a triarylamine
derivative.
The above-described materials used in a hole injection layer may be
used singly or plural kinds may be co-used.
<<Additive>>
The above-described organic layer of the present invention may
further contain other additive.
Examples of an additive are: halogen elements such as bromine,
iodine and chlorine, and a halide compound; and a compound, a
complex and a salt of an alkali metal, an alkaline earth metal and
a transition metal such as Pd, Ca and Na.
Although a content of an additive may be arbitrarily decided,
preferably, it is 1,000 ppm or less based on the total mass of the
layer containing the additive, more preferably, it is 500 ppm or
less, and still more preferably, it is 50 ppm or less.
In order to improve a transporting porporty of an electron or a
hole, or to facilitate energy transport of an exciton, the content
of the additive is not necessarily within these range, and other
range of content may be used.
<<Forming Method of Organic Layers>>
It will be described forming methods of organic layers according to
the present invention (hole injection layer, hole transport layer,
light emitting layer, hole blocking layer, electron transport
layer, and electron injection layer).
Forming methods of organic layers according to the present
invention are not specifically limited. They may be formed by using
a known method such as a vacuum vapor deposition method and a wet
method (wet process).
Examples of a wet process include: a spin coating method, a cast
method, an inkjet method, a printing method, a die coating method,
a blade coating method, a roll coating method, a spray coating
method, a curtain coating method, and a LB method (Langmuir
Blodgett method). From the viewpoint of getting a uniform thin
layer with high productivity, preferable are method highly
appropriate to a roll-to-roll method such as a die coating method,
a roll coating method, an inkjet method, and a spray coating
method.
Examples of a liquid medium to dissolve or to disperse a material
for organic layers according to the present invention include:
ketones such as methyl ethyl ketone and cyclohexanone; aliphatic
esters such as ethyl acetate; halogenated hydrocarbons such as
dichlorobenzene; aromatic hydrocarbons such as toluene, xylene,
mesitylene, and cyclohexylbenzene; aliphatic hydrocarbons such as
cyclohexane, decalin, and dodecane; organic solvents such as DMF
and DMSO.
These will be dispersed with a dispersion method such as an
ultrasonic dispersion method, a high shearing dispersion method and
a media dispersion method.
A different film forming method may be applied to every organic
layer. When a vapor deposition method is adopted for forming each
layer, the vapor deposition conditions may be changed depending on
the compounds used. Generally, the following ranges are suitably
selected for the conditions, heating temperature of boat: 50 to
450.degree. C., level of vacuum: 1.times.10.sup.-6 to
1.times.10.sup.-2 Pa, vapor deposition rate: 0.01 to 50 nm/sec,
temperature of substrate: -50 to 300.degree. C., and layer
thickness: 0.1 nm to 5 .mu.m, preferably 5 to 200 nm.
Formation of organic layers of the present invention is preferably
continuously carried out from a hole injection layer to a cathode
with one time vacuuming. It may be taken out on the way, and a
different layer forming method may be employed. In that case, the
operation is preferably done under a dry inert gas atmosphere.
<<Anode>>
As an anode of an organic EL element, a metal having a large work
function (4 eV or more, preferably, 4.5 eV or more), an alloy, and
a conductive compound and a mixture thereof are utilized as an
electrode substance.
Specific examples of an electrode substance are: metals such as Au,
and an alloy thereof; transparent conductive materials such as CuI,
indium tin oxide (ITO), SnO.sub.2, and ZnO. Further, a material
such as IDIXO (In.sub.2O.sub.3--ZnO), which may form an amorphous
and transparent electrode, may also be used.
As for an anode, these electrode substances may be made into a thin
layer by a method such as a vapor deposition method or a sputtering
method; followed by making a pattern of a desired form by a
photolithography method. Otherwise, when the requirement of pattern
precision is not so severe (about 100 .mu.m or more), a pattern may
be formed through a mask of a desired form at the time of layer
formation with a vapor deposition method or a sputtering method
using the above-described material.
Alternatively, when a coatable substance such as an organic
conductive compound is employed, it is possible to employ a wet
film forming method such as a printing method or a coating method.
When emitted light is taken out from the anode, the transmittance
is preferably set to be 10% or more. A sheet resistance of the
anode is preferably a few hundred .OMEGA./sq or less.
Further, although a layer thickness of the anode depends on a
material, it is generally selected in the range of 10 nm to 1
.mu.m, and preferably in the range of 10 to 200 nm.
<<Cathode>>
As a cathode, a metal having a small work function (4 eV or less)
(it is called as an electron injective metal), an alloy, a
conductive compound and a mixture thereof are utilized as an
electrode substance. Specific examples of the aforesaid electrode
substance includes: sodium, sodium-potassium alloy, magnesium,
lithium, a magnesium/copper mixture, a magnesium/silver mixture, a
magnesium/aluminum mixture, a magnesium/indium mixture, an
aluminum/aluminum oxide (Al.sub.2O.sub.3) mixture, indium, a
lithium/aluminum mixture, aluminum, and a rare earth metal. Among
them, with respect to an electron injection property and durability
against oxidation, preferable are: a mixture of election injecting
metal with a second metal which is stable metal having a work
function larger than the electron injecting metal. Examples thereof
are: a magnesium/silver mixture, a magnesium/aluminum mixture, a
magnesium/indium mixture, an aluminum/aluminum oxide
(Al.sub.2O.sub.3) mixture, a lithium/aluminum mixture and
aluminum.
A cathode may be made by using these electrode substances with a
method such as a vapor deposition method or a sputtering method to
form a thin film. A sheet resistance of the cathode is preferably a
few hundred Q/sq or less. A layer thickness of the cathode is
generally selected in the range of 10 nm to 5 .mu.m, and preferably
in the range of 50 to 200 nm.
In order to transmit emitted light, it is preferable that one of an
anode and a cathode of an organic EL element is transparent or
translucent for achieving an improved luminescence.
Further, after forming a layer of the aforesaid metal having a
thickness of 1 to 20 nm on the cathode, it is possible to prepare a
transparent or translucent cathode by providing with a conductive
transparent material described in the description for the anode
thereon. By applying this process, it is possible to produce an
element in which both an anode and a cathode are transparent.
[Support Substrate]
A support substrate which may be used for an organic EL element of
the present invention is not specifically limited with respect to
types such as glass and plastics. Hereafter, the support substrate
may be also called as substrate body, substrate, substrate
substance, or support. They may be transparent or opaque. However,
a transparent support substrate is preferable when the emitting
light is taken from the side of the support substrate. Support
substrates preferably utilized includes such as glass, quartz and
transparent resin film. A specifically preferable support substrate
is a resin film capable of providing an organic EL element with a
flexible property.
Examples of a resin film include: polyesters such as polyethylene
terephthalate (PET) and polyethylene naphthalate (PEN),
polyethylene, polypropylene, cellophane, cellulose esters and their
derivatives such as cellulose diacetate, cellulose triacetate
(TAC), cellulose acetate butyrate, cellulose acetate propionate
(CAP), cellulose acetate phthalate, and cellulose nitrate,
polyvinylidene chloride, polyvinyl alcohol, polyethylene vinyl
alcohol, syndiotactic polystyrene, polycarbonate, norbornene resin,
polymethyl pentene, polyether ketone, polyimide, polyether sulfone
(PES), polyphenylene sulfide, polysulfones, polyether imide,
polyether ketone imide, polyamide, fluororesin, Nylon, polymethyl
methacrylate, acrylic resin, polyallylates and cycloolefin resins
such as ARTON (trade name, made by JSR Co. Ltd.) and APEL (trade
name, made by Mitsui Chemicals, Inc.).
On the surface of a resin film, it may be formed a film
incorporating an inorganic or an organic compound or a hybrid film
incorporating both compounds. Barrier films are preferred with a
water vapor permeability of 0.01 g/m.sup.224 h or less (at
25.+-.0.5.degree. C., and 90.+-.2% RH) determined based on JIS
K7129-1992. Further, high barrier films are preferred to have an
oxygen permeability of 1.times.10.sup.-3 cm.sup.3/m.sup.224 hatm or
less determined based on JIS K 7126-1987, and a water vapor
permeability 1 of 1.times.10.sup.-5 g/m.sup.224 h or less.
As materials that form a barrier film, employed may be those which
retard penetration of moisture and oxygen, which deteriorate the
element. For example, it is possible to employ silicon oxide,
silicon dioxide, and silicon nitride. Further, in order to improve
the brittleness of the aforesaid film, it is more preferable to
achieve a laminated layer structure of inorganic layers and organic
layers. The laminating order of the inorganic layer and the organic
layer is not particularly limited, but it is preferable that both
are alternatively laminated a plurality of times.
Barrier film forming methods are not particularly limited, and
examples of employable methods include a vacuum deposition method,
a sputtering method, a reactive sputtering method, a molecular beam
epitaxy method, a cluster ion beam method, an ion plating method, a
plasma polymerization method, a plasma CVD method, a laser CVD
method, a thermal CVD method, and a coating method. Of these,
specifically preferred is a method employing an atmospheric
pressure plasma polymerization method, described in JP-A No.
2004-68143.
Examples of opaque support substrates include metal plates such
aluminum or stainless steel films, opaque resin substrates, and
ceramic substrates.
An external taking out quantum efficiency of light emitted by the
organic EL element of the present invention is preferably at least
1% at a room temperature, but is more preferably at least 5%.
External taking out quantum efficiency (%)=(Number of photons
emitted by the organic EL element to the exterior/Number of
electrons fed to organic EL element).times.100.
Further, it may be used simultaneously a color hue improving filter
such as a color filter, or it may be used simultaneously a color
conversion filter which convert emitted light color from the
organic EL element to multicolor by employing fluorescent
materials.
[Sealing]
As sealing means employed in the present invention, listed may be,
for example, a method in which sealing members, electrodes, and a
supporting substrate are subjected to adhesion via adhesives. The
sealing members may be arranged to cover the display region of an
organic EL element, and may be a concave plate or a flat plate.
Neither transparency nor electrical insulation is limited.
Specifically listed are glass plates, polymer plate-films, metal
plate-films. Specifically, it is possible to list, as glass plates,
soda-lime glass, barium-strontium containing glass, lead glass,
aluminosilicate glass, borosilicate glass, barium borosilicate
glass, and quartz. Further, listed as polymer plates may be
polycarbonate, acryl, polyethylene terephthalate, polyether
sulfide, and polysulfone. As a metal plate, listed are those
composed of at least one metal selected from the group consisting
of stainless steel, iron, copper, aluminum magnesium, nickel, zinc,
chromium, titanium, molybdenum, silicon, germanium, and tantalum,
or alloys thereof.
In the present invention, since it is possible to achieve a thin
organic EL element, it is preferable to employ a polymer film or a
metal film. Further, it is preferable that the polymer film has an
oxygen permeability of 1.times.10.sup.-3 cm.sup.3/m.sup.224 hatm or
less determined by the method based on JIS K 7126-1987, and a water
vapor permeability of 1.times.10.sup.-3 g/m.sup.224 h or less (at
25.+-.0.5.degree. C., and 90.+-.2% RH) determined by the method
based on JIS K 7129-1992.
Conversion of the sealing member into concave is carried out by
employing a sand blast process or a chemical etching process.
In practice, as adhesives, listed may be photo-curing and
heat-curing types having a reactive vinyl group of acrylic acid
based oligomers and methacrylic acid, as well as moisture curing
types such as 2-cyanoacrylates. Further listed may be thermal and
chemical curing types (mixtures of two liquids) such as epoxy based
ones. Still further listed may be hot-melt type polyamides,
polyesters, and polyolefins. Yet further listed may be cationically
curable type UV curable epoxy resin adhesives.
In addition, since an organic EL element is occasionally
deteriorated via a thermal process, preferred are those which
enable adhesion and curing between a room temperature and
80.degree. C. Further, desiccating agents may be dispersed into the
aforesaid adhesives. Adhesives may be applied onto sealing portions
via a commercial dispenser or printed on the same in the same
manner as screen printing.
Further, it is appropriate that on the outside of the aforesaid
electrode which interposes the organic layer and faces the support
substrate, the aforesaid electrode and organic layer are covered,
and in the form of contact with the support substrate, inorganic
and organic material layers are formed as a sealing film. In this
case, as materials that form the aforesaid film may be those which
exhibit functions to retard penetration of moisture or oxygen which
results in deterioration. For example, it is possible to employ
silicon oxide, silicon dioxide, and silicon nitride.
Still further, in order to improve brittleness of the aforesaid
film, it is preferable that a laminated layer structure is formed,
which is composed of these inorganic layers and layers composed of
organic materials. Methods to form these films are not particularly
limited. It is possible to employ, for example, a vacuum deposition
method, a sputtering method, a reactive sputtering method, a
molecular beam epitaxy method, a cluster ion beam method, an ion
plating method, a plasma polymerization method, an atmospheric
pressure plasma polymerization method, a plasma CVD method, a
thermal CVD method, and a coating method.
It is preferable to inject a gas phase and a liquid phase material
of inert gases such as nitrogen or argon, and inactive liquids such
as fluorinated hydrocarbon or silicone oil into the space between
the space formed with the sealing member and the display region of
the organic EL element. Further, it is possible to form vacuum in
the space. Still further, it is possible to enclose hygroscopic
compounds in the interior of the space.
Examples of a hygroscopic compound include: metal oxides (for
example, sodium oxide, potassium oxide, calcium oxide, barium
oxide, magnesium oxide, and aluminum oxide); sulfates (for example,
sodium sulfate, calcium sulfate, magnesium sulfate, and cobalt
sulfate); metal halides (for example, calcium chloride, magnesium
chloride, cesium fluoride, tantalum fluoride, cerium bromide,
magnesium bromide, barium iodide, and magnesium iodide);
perchlorates (for example, barium perchlorate and magnesium
perchlorate). In sulfates, metal halides, and perchlorates,
suitably employed are anhydrides. For sulfate salts, metal halides
and perchlorates, suitably used are anhydrous salts.
[Protective Film and Protective Plate]
On the aforesaid sealing film which interposes the organic layer
and faces the support substrate or on the outside of the aforesaid
sealing film, a protective or a protective plate may be arranged to
enhance the mechanical strength of the element. Specifically, when
sealing is achieved via the aforesaid sealing film, the resulting
mechanical strength is not always high enough, therefore it is
preferable to arrange the protective film or the protective plate
described above. Usable materials for these include glass plates,
polymer plate-films, and metal plate-films which are similar to
those employed for the aforesaid sealing. However, from the
viewpoint of reducing weight and thickness, it is preferable to
employ a polymer film.
[Improving Method of Light Extraction]
It is generally known that an organic EL element emits light in the
interior of the layer exhibiting the refractive index (being about
1.6 to 2.1) which is greater than that of air, whereby only about
15% to 20% of light generated in the light emitting layer is
extracted. This is due to the fact that light incident to an
interface (being an interlace of a transparent substrate to air) at
an angle of .theta. which is at least critical angle is not
extracted to the exterior of the element due to the resulting total
reflection, or light is totally reflected between the transparent
electrode or the light emitting layer and the transparent
substrate, and light is guided via the transparent electrode or the
light emitting layer, whereby light escapes in the direction of the
element side surface.
Means to enhance the efficiency of the aforesaid light extraction
include, for example: a method in which roughness is formed on the
surface of a transparent substrate, whereby total reflection is
minimized at the interface of the transparent substrate to air
(U.S. Pat. No. 4,774,435), a method in which efficiency is enhanced
in such a manner that a substrate results in light collection (JP-A
No. 63-314795), a method in which a reflection surface is formed on
the side of the element (JP-A No. 1-220394), a method in which a
flat layer of a middle refractive index is introduced between the
substrate and the light emitting body and an antireflection film is
formed (JP-A No. 62-172691), a method in which a flat layer of a
refractive index which is equal to or less than the substrate is
introduced between the substrate and the light emitting body (JP-A
No. 2001-202827), and a method in which a diffraction grating is
formed between the substrate and any of the layers such as the
transparent electrode layer or the light emitting layer (including
between the substrate and the outside) (JP-A No. 11-283751).
In the present invention, it is possible to employ these methods
while combined with the organic EL element of the present
invention. Of these, it is possible to appropriately employ the
method in which a flat layer of a refractive index which is equal
to or less than the substrate is introduced between the substrate
and the light emitting body and the method in which a diffraction
grating is formed between any layers of a substrate, and a
transparent electrode layer and a light emitting layer (including
between the substrate and the outside space).
By combining these means, the present invention enables the
production of elements which exhibit higher luminance or excel in
durability.
When a low refractive index medium having a thickness, greater than
the wavelength of light is formed between the transparent electrode
and the transparent substrate, the extraction efficiency of light
emitted from the transparent electrode to the exterior increases as
the refractive index of the medium decreases.
As materials of the low refractive index layer, listed are, for
example, aerogel, porous silica, magnesium fluoride, and fluorine
based polymers. Since the refractive index of the transparent
substrate is commonly about 1.5 to 1.7, the refractive index of the
low refractive index layer is preferably approximately 1.5 or less.
More preferably, it is 1.35 or less.
Further, thickness of the low refractive index medium is preferably
at least two times of the wavelength in the medium. The reason is
that, when the thickness of the low refractive index medium reaches
nearly the wavelength of light so that electromagnetic waves
escaped via evanescent enter into the substrate, effects of the low
refractive index layer are lowered.
The method in which the interface which results in total reflection
or a diffraction grating is introduced in any of the media is
characterized in that light extraction efficiency is significantly
enhanced. The above method works as follows. By utilizing
properties of the diffraction grating capable of changing the light
direction to the specific direction different from diffraction via
so-called Bragg diffraction such as primary diffraction or
secondary diffraction of the diffraction grating, of light emitted
from the light entitling layer, light, which is not emitted to the
exterior due to total reflection between layers, is diffracted via
introduction of a diffraction grating between any layers or in a
medium (in the transparent substrate and the transparent electrode)
so that light is extracted to the exterior.
It is preferable that the introduced diffraction grating exhibits a
two-dimensional periodic refractive index. The reason is as
follows. Since light emitted in the light emitting layer is
randomly generated to all directions, in a common one-dimensional
diffraction grating exhibiting a periodic refractive index
distribution only in a certain direction, light which travels to
the specific direction is only diffracted, whereby light extraction
efficiency is not sufficiently enhanced.
However, by changing the refractive index distribution to a
two-dimensional one, light, which travels to all directions, is
diffracted, whereby the light extraction efficiency is
enhanced.
A position to introduce a diffraction grating may be between any
layers or in a medium (in a transparent substrate or a transparent
electrode). However, a position near the organic light emitting
layer, where light is generated, is preferable. In this case, the
cycle of the diffraction grating is preferably from about 1/2 to 3
times of the wavelength of light in the medium. The preferable
arrangement of the diffraction grating is such that the arrangement
is two-dimensionally repeated in the form of a square lattice, a
triangular lattice, or a honeycomb lattice.
[Light Collection Sheet]
Via a process to arrange a structure such as a micro-lens array
shape on the light extraction side of the organic EL element of the
present invention or via combination with a so-called light
collection sheet, light is collected in the specific direction such
as the front direction with respect to the light emitting element
surface, whereby it is possible to enhance luminance in the
specific direction.
In an example of the micro-lens array, square pyramids to realize a
side length of 30 .mu.m and an apex angle of 90 degrees are
two-dimensionally arranged on the light extraction side of the
substrate. The side length is preferably 10 to 100 .mu.m. When it
is less than the lower limit, coloration occurs due to generation
of diffraction effects, while when it exceeds the upper limit, the
thickness increases undesirably.
It is possible to employ, as a light collection sheet, for example,
one which is put into practical use in the LED backlight of liquid
crystal display devices. It is possible to employ, as such a sheet,
for example, the luminance enhancing film (BEF), produced by
Sumitomo 3M Limited. As shapes of a prism sheet employed may be,
for example, A shaped stripes of an apex angle of 90 degrees and a
pitch of 50 .mu.m formed on a base material, a shape in which the
apex angle is rounded, a shape in which the pitch is randomly
changed, and other shapes.
Further, in order to control the light radiation angle from the
light emitting element, simultaneously employed may be a light
diffusion plate-film. For example, it is possible to employ the
diffusion film (LIGHT-UP), produced by Kimoto Co., Ltd.
[Applications]
It is possible to employ the organic EL element of the present
invention as display devices, displays, and various types of light
emitting sources.
Examples of light emitting sources include: lighting apparatuses
(home lighting and car lighting), clocks, backlights for liquid
crystals, sign advertisements, signals, light sources of light
memory media, light sources of electrophotographic copiers, light
sources of light communication processors, and light sources of
light sensors. The present invention is not limited to them. It is
especially effectively employed as a backlight of a liquid crystal
display device and a lighting source.
If needed, the organic EL element of the present, invention may
undergo patterning via a metal mask or an ink-jet printing method
during film formation. When the patterning is carried out, only an
electrode may undergo patterning, an electrode and a light emitting
layer may undergo patterning, or all element layers may undergo
patterning. During preparation of the element, it is possible to
employ conventional methods.
<Display Device>
A display device provided with an organic EL element of the present
invention may emit a single color or multiple colors. Here, it will
be described a multiple color display device.
In case of a multiple color display device, a shadow mask is placed
during the formation of a light emitting layer, and a layer is
formed as a whole with a vapor deposition method, a cast method, a
spin coating method, an inkjet method, and a printing method.
When patterning is done only to the light emitting layer, although
the coating method is not limited in particular, preferable methods
are a vapor deposition method, an inkjet method, a spin coating
method, and a printing method.
A constitution of an organic EL element provided for a display
device is selected from the above-described examples of an organic
EL element according to the necessity.
The production method of an organic EL element is described as an
embodiment of a production method of the above-described organic EL
element.
When a direct-current voltage is applied to the produced multiple
color display device, light emission may be observed by applying
voltage of 2.largecircle.40 V by setting the anode to have a plus
(+) polarity, and the cathode to have a minus (-) polarity. When
the voltage is applied to the device with reverse polarities, an
electric current does not pass and light emission does not occur.
Further, when an alternating-current voltage is applied to the
device, light emission occurs only when the anode has a plus (+)
polarity and the cathode has a minus (-) polarity. In addition, an
arbitrary wave shape may be used for applying
alternating-current.
The multiple color display device may be used for a display device,
a display, and a variety of light emitting sources. In a display
device or a display, a full color display is possible by using 3
kinds of organic EL elements emitting blue, red and green.
Examples of a display device or a display are: a television set, a
personal computer, a mobile device, an AV device, a character
broadcast display, and an information display in a car.
Specifically, it may be used for a display device reproducing a
still image or a moving image. When it is used for a display device
reproducing a moving image, the driving mode may be any one of a
passive-matrix mode and an active-matrix mode.
Examples of a light emitting source include: home lighting, car
lighting, backlights for clocks and liquid crystals, sign
advertisements, signals, light sources of light memory media, light
sources of electrophotographic copiers, light sources of light
communication processors, and light sources of light sensors. The
present invention is not limited to them.
In the following, an example of a display device provided with an
organic EL element of the present invention will be described by
referring to drawings.
FIG. 9 is a schematic drawing illustrating an example of a display
device composed of an organic EL element. Display of image
information is carried out by light emission of an organic EL
element. For example, it is a schematic drawing of a display of a
cell-phone.
A display 1 is constituted of a display section A having plural
number of pixels, a control section B which performs image scanning
of the display section A based on image information, and a wiring
section C electrically connecting the display section A and the
control section B.
The control section B, which is electrically connected to the
display section A via the wiring section C, sends a scanning signal
and an image data signal to plural number of pixels based on image
information from the outside and pixels of each scanning line
successively emit depending on the image data signal by a scanning
signal to perform image scanning, whereby image information is
displayed on the display section A.
FIG. 10 is a schematic drawing of the display section A based on an
active matrix mode.
The display section A is provided with the wiring section C, which
contains plural scanning lines 5 and data lines 6, and plural
pixels 3 on a substrate. Primary part materials of the display
section A will be explained in the following.
In FIG. 10, illustrated is the case that light emitted by the pixel
3 is taken out along the white allow (downward).
The scanning lines 5 and the plural data lines 6 each are comprised
of a conductive material, and the scanning lines 5 and the data
lines 6 are perpendicular in a grid form and are connected to
pixels 3 at the right-angled crossing points (details are not shown
in the drawing).
The pixel 3 receives an image data from the data line 6 when a
scanning signal is applied from the scanning line 5 and emits
according to the received image data.
Full-color display is possible by appropriately arranging pixels
having an emission color in a red region, pixels in a green region
and pixels in a blue region, side by side on the same
substrate.
Next, an emission process of a pixel will be explained. FIG. 11 is
a schematic drawing of a pixel.
A pixel is equipped with an organic EL element 10, a switching
transistor 11, an operating transistor 12 and a capacitor 13. Red,
green and blue emitting organic EL elements are utilized as the
organic EL element 10 for plural pixels, and full-color display
device is possible by arranging these side by side on the same
substrate.
In FIG. 11, an image data signal is applied on the drain of the
switching transistor 11 via the data line 6 from the control
section B. Then when a scanning signal is applied on the gate of
the switching transistor 11 via the scanning line 5 from control
section B, operation of switching transistor is on to transmit the
image data signal applied on the drain to the gates of the
capacitor 13 and the operating transistor 12.
The operating transistor 12 is on, simultaneously with the
capacitor 13 being charged depending on the potential of an image
data signal, by transmission of an image data signal. In the
operating transistor 12, the drain is connected to an electric
source line 7 and the source is connected to the electrode of the
organic EL element 10, and an electric current is supplied from the
electric source line 7 to the organic EL element 10 depending on
the potential of an image data applied on the gate.
When a scanning signal is transferred to the next scanning line 5
by successive scanning of the control section B, operation of the
switching transistor 11 is off.
However, since the capacitor 13 keeps the charged potential of an
image data signal even when operation of the switching transistor
11 is off, operation of the operating transistor 12 is kept on to
continue emission of the organic EL element 10 until the next
scanning signal is applied.
When the next scanning signal is applied by successive scanning,
the operating transistor 12 operates depending on the potential of
an image data signal synchronized to the scanning signal and the
organic EL element 10 emits light.
That is, emission of each organic EL element 10 of the plural
pixels 3 is performed by providing the switching transistor 11 and
the operating transistor 12 against each organic EL element 10 of
plural pixels 3. Such an emission method is called as an active
matrix mode.
Herein, emission of the organic EL element 10 may be either
emission of plural gradations based on a multiple-valued image data
signal having plural number of gradation potentials or on and off
of a predetermined emission quantity based on a binary image data
signal. Further, potential hold of the capacitor 13 may be either
continuously maintained until the next scanning signal application
or discharged immediately before the next scanning signal
application.
In the present invention, emission operation is not necessarily
limited to the above-described active matrix mode but may be a
passive matrix mode in which organic EL element is emitted based on
a data signal only when a scanning signal is scanned.
FIG. 12 is a schematic drawing of a display device based on a
passive matrix mode. In FIG. 12, plural number of scanning lines 5
and plural number of image data lines 6 are arranged grid-wise,
opposing to each other and sandwiching the pixels 3.
When a scanning signal of the scanning line 5 is applied by
successive scanning, the pixel 3 connected to the scanning line 5
applied with the signal emits depending on an image data
signal.
Since the pixel 3 is provided with no active element in a passive
matrix mode, decrease of manufacturing cost is possible.
By employing the organic EL element of the present invention, it
was possible to obtain a display device having improved emission
efficiency.
<Light Emitting Device>
An organic EL element of the present invention may be used for a
light emitting device.
An organic EL element of the present invention may be provided with
a rasonator structure. The intended uses of the organic EL element
provided with a rasonator structure are: a light source of a light
memory media, a light source of an electrophotographic copier, a
light source of a light communication processor, and a light source
of a light sensor, however, it is not limited to them. It may be
used for the above-described purposes by making to emit a
laser.
Further, an organic EL element of the present invention may be used
for a kind of lamp such as for illumination or exposure. It may be
used for a projection device for projecting an image, or may be
used for a display device to directly observe a still image or a
moving image thereon.
The driving mode used for a display device of a moving image
reproduction may be any one of a passive matrix mode and an active
matrix mode. By employing two or more kinds of organic EL elements
of the present invention emitting a different emission color, it
may produce a full color display device.
In addition, a .pi.-conjugated compound used in the present
invention may be applicable to an organic EL element substantially
emitting white light as a light emitting device. For example, when
a plurality of light emitting materials are employed, white light
may be obtained by mixing colors of a plurality of emission colors.
As a combination of the plurality of emission colors, it may be a
combination of red, green and blue having emission maximum
wavelength of three primary colors, or it may be a combination of
colors having two emission maximum wavelength making use of the
relationship of two complementary colors of blue and yellow, or
blue-green and orange.
A production method of an organic EL element of the present
invention is done by placing a mask only during formation of a
light emitting layer, a hole transport layer and an electron
transport layer. It may be produced by coating with a mask to make
simple arrangement. Since other layers are common, there is no need
of pattering with a mask. For example, it may produce an electrode
uniformly with a vapor deposition method, a cast method, a spin
coating method, an inkjet method, and a printing method. The
production yield will be improved.
By using these methods, it may be produced a white organic EL
device in which a plurality of light emitting elements are arranged
in parallel to form an array state. The element itself emits white
light.
[One Embodiment of Lighting Device of the Present Invention]
One embodiment of lighting devices of the present Invention
provided with an organic EL element of the present invention will
be described.
The non-light emitting surface of the organic EL element of the
present invention was covered with a glass case, and a 300 .mu.m
thick glass substrate was employed as a sealing substrate. An epoxy
based light curable type adhesive (LUXTRACK LC0629B produced by
Toagosei Co., Ltd.) was employed in the periphery as a sealing
material. The resulting one was superimposed on the aforesaid
cathode to be brought into close contact with the aforesaid
transparent support substrate, and curing and sealing were carried
out via exposure of UV radiation onto the glass substrate side,
whereby the lighting device shown in FIG. 13 and FIG. 14, was
formed.
FIG. 13 is a schematic view of a lighting device, and an organic EL
element of the present invention (Organic EL element 101 in a light
emitting device) is covered with glass cover 102 (incidentally,
sealing by the glass cover was carried out in a globe box under
nitrogen ambience (under an ambience of high purity nitrogen gas at
a purity of at least 99.999%) so that Organic EL Element 101 was
not brought into contact with atmosphere).
FIG. 14 is a cross-sectional view of a lighting device. In FIG. 6,
105 represents a cathode, 106 represents an organic EL layer, and
107 represents a glass substrate fitted with a transparent
electrode. Further, the interior of glass cover 102 is filled with
nitrogen gas 108 and water catching agent 109 is provided.
By employing an organic EL element of the present invention, it was
possible to obtain a light emitting having improved emission
efficiency.
<Light-emitting Thin Film>
A light-emitting thin film of the present invention is
characterized in containing a .pi.-conjugated compound according to
the above-described present invention. It may be produced in the
same way as preparation of the above-described organic layer.
Forming methods of a light-emitting thin film according to the
present invention are not specifically limited. They may be formed
by using a known method such as a vacuum vapor deposition method
and a wet method (wet process).
Examples of a wet process include: a spin coating method, a cast
method, an inkjet method, a printing method, a die coating method,
a blade coating method, a roll coating method, a spray coating
method, a curtain coating method, and a LB method (Langmuir
Blodgett method). From the viewpoint of getting a uniform thin
layer with high productivity, preferable are methods highly
appropriate to a roll-to-roll method such as a die coating method,
a roll coating method, an inkjet method, and a spray coating
method.
Examples of a liquid medium to dissolve or to disperse a
.pi.-conjugated compound according to the present invention
include: ketones such as methyl ethyl ketone and cyclohexanone;
aliphatic esters such as ethyl acetate; halogenated hydrocarbons
such as dichlorobenzene; aromatic hydrocarbons such as toluene,
xylene, mesitylene, and cyclohexylbenzene; aliphatic hydrocarbons
such as cyclohexane, decalin, and dodecane; organic solvents such
as DMF and DMSO.
These will be dispersed with a dispersion method such as an
ultrasonic dispersion method, a high shearing dispersion method and
a media dispersion method.
A different film forming method may be applied to every organic
layer. When a vapor deposition method is adopted for forming each
layer, the vapor deposition conditions will change depending on the
compounds used. Generally, the following ranges are suitably
selected for the conditions, heating temperature of boat: 50 to
450.degree. C., level of vacuum: 1.times.10.sup.-6 to
1.times.10.sup.-2 Pa, vapor deposition rate: 0.01 to 50 nm/sec,
temperature of substrate: -50 to 300.degree. C., and layer
thickness: 0.1 nm to 5 .mu.m, preferably 5 to 200 nm.
When a spin coating method is adopted, it is preferable to use a
spin coater in the range of 100 to 1000 rpm for 10 to 120 seconds
under a dry inert gas atmosphere.
A light-emitting thin film of the present invention may be used for
a display device or a light emitting device.
EXAMPLES
Hereafter, the present invention will be described specifically by
referring to examples, however, the present invention is not
limited to them. In examples, the indication of "%" is used. Unless
particularly mentioned, it represents "mass %".
A calculation method of an angle .theta. in Examples will be
described by referring to a compound T-93 as an example. The
compound T-93 was calculated by performing an optimization of a
structure with a density-functional calculation method using B3LYP
as a functional and 6-31G(d) as a base function. The theoretical
calculation results revealed that a LUMO is localized at a
dicyanobenzene being an acceptor portion, and a HOMO is localized
at a 9,10-dihydroacrydine portion being a donor portion. Since the
frontier orbitals of the compound T-93 are a .pi.*-orbital and a
.pi.-orbital, as described above, a vertical direction to the
.pi.-conjugated plane is a direction vector of the LUMO orbital and
the HOMO orbital defined in the present invention (refer to FIG.
15). By using the optimized structure obtained by the theoretical
calculation, the angle .theta. formed with two direction vectors
was calculated to be 164 degree for the compound T-93.
Example 1
(Preparation of Organic EL Element 1-1)
An anode was prepared on a glass substrate of 50 mm.times.50 mm
with a thickness of 0.7 mm by forming a film of ITO (indium tin
oxide) with a thickness of 150 nm, then by making patterning to it.
The transparent support substrate provided with the ITO transparent
electrode was subjected to ultrasonic washing with isopropyl
alcohol, followed by drying with desiccated nitrogen gas, and it
was subjected to UV ozone washing for 5 minutes. The transparent
support substrate was fixed to a substrate holder of a commercial
vacuum deposition apparatus.
The constituting materials for each layer were loaded in each
heating boat for vapor deposition in the vacuum deposition
apparatus with an optimum amount. As a heating boat for vapor
deposition, it was used a resistance heating boat made of
molybdenum or tungsten.
After reducing the pressure of a vacuum tank to 4.times.10.sup.-4
Pa, the heating boat containing HAT-CN
(1,4,5,8,9,12-hexaazatriphenylene hexacarbonitrile) was heated via
application of electric current and vapor deposition was made onto
the ITO transparent electrode at a deposition rate of 0.1
nm/second, whereby it was produced a hole injection layer having a
thickness of 10 nm.
Subsequently, .alpha.-NPD
(4,4'-bis[N-(naphthyl)-N-phenylamino]biphenyl) was deposited onto
the hole injection layer at a deposition rate of 0.1 nm/second,
whereby it was produced a hole transport layer having a thickness
of 40 nm.
Further, a host compound mCP (1,3-bis(N-carbazolyl) benzene) and a
comparative compound 1 were co-deposited onto the hole transport
layer at a deposition rate of 0.1 nm/second so that they have 96
volume % and 4 volume % respectively, whereby it was produced a
light emitting layer having a thickness of 30 nm.
Subsequently, BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)
was deposited at a deposition rate of 0.1 nm/second, whereby it was
produced an electron transport layer having a thickness of 30
nm.
Further, after forming a lithium fluoride layer having a thickness
of 0.5 nm, 110 nm thick aluminum was vapor deposited to form a
cathode.
The non-light emitting surface side of the produced element was
sealed by a glass case having a can shape under an ambience of high
purity nitrogen gas having a purity of at least 99.999%. The
electrode taken out wiring was set to obtain an organic EL element
1-1.
##STR00026## (Preparation of Organic EL Elements 1-2 to 1-9)
Organic EL elements 1-2 to 1-9 were prepared in the same manner as
preparation of the organic EL element 1-1 except that the light
emitting compound was changed from the comparative compound 1 to
the compounds described in Table 1.
TABLE-US-00001 TABLE 1 Emission Organic EL Emission Angle .theta.
.DELTA.E.sub.ST efficiency element No. compound (degree) (eV) (%)
Remarks 1-1 Comparative 50 0.47 100 Comp. compound 1 1-2 T-71 130
0.18 109 Inv. 1-3 T-19 135 0.02 114 Inv. 1-4 T-2 120 0.04 111 Inv.
1-5 T-86 141 0.03 117 Inv. 1-6 T-92 110 0.03 111 Inv. 1-7 T-93 164
0.01 121 Inv. 1-8 T-96 113 0.04 110 Inv. 1-9 Comparative 71 0.07
103 Comp. compound 2 Comp.: Comparative example Inv.: Inventive
example
Example 2
(Preparation of Organic EL element 2-1)
An anode was prepared by making patterning to a glass substrate of
100 mm.times.100 mm.times.1.1 mm (NA45, produced by NH Techno Glass
Corp.) on which ITO (indium tin oxide) was formed with a thickness
of 100 nm. Thereafter, the above transparent support substrate
provided with the ITO transparent electrode was subjected to
ultrasonic washing with isopropyl alcohol, followed by drying with
desiccated nitrogen gas, and it was subjected to UV ozone washing
for 5 minutes.
On the transparent support substrate thus prepared was applied a
70% solution of poly (3,4-ethylenedioxythiphene)-polystyrene
sulfonate (PEDOT/PSS, Baytron P AI4083, made by Bayer AG.) diluted
with water by using a spin coating method at 3,000 rpm for 30
seconds to forma film, and then it was dried at 200.degree. C. for
one hour. A hole injection layer having a thickness of 20 nm was
prepared. The resulting transparent support substrate was fixed to
a substrate holder of a commercial vacuum deposition apparatus. The
constituting materials for each layer were loaded in each heating
boat for vapor deposition in the vacuum deposition apparatus with
an optimum amount. As a heating boat for vapor deposition, it was
used a resistance heating boat made of molybdenum or tungsten.
After reducing the pressure of a vacuum tank to 4.times.10.sup.-4
Pa, .alpha.-NPD was deposited onto the hole injection layer at a
deposition rate of 0.1 nm/second, whereby it was produced a hole
transport layer having a thickness of 40 nm.
CDBP and perylene were co-deposited onto the hole transport layer
at a deposition rate of 0.1 nm/second so that they have 94 volume %
and 6 volume % respectively, whereby it was produced a light
emitting layer having a thickness of 30 nm.
Subsequently, TPBi (1,3,5-tris(N-benzimidazole-2-yl)) was deposited
at a deposition rate of 0.1 nm/second, whereby it was produced an
electron transport layer having a thickness of 30 nm.
Further, after forming a lithium fluoride layer having a thickness
of 0.5 nm, 110 nm thick aluminum was vapor deposited to form a
cathode.
The non-light emitting surface side of the produced element was
sealed by a glass case having a can shape under an ambience of high
purity nitrogen gas having a purity of at least 99.999%. The
electrode taken out wiring was set to obtain an organic EL element
2-1.
(Preparation of Organic EL Element 2-2)
Organic EL elements 2-2 was prepared in the same manner as
preparation of the organic EL element 2-1 except that the light
emitting layer was formed by using: CDBP as a host compound;
perylene as a light emitting compound; and the comparative compound
1 as a third compound, and the contents of compounds were
respectively adjusted to be 80 volume %, 6 volume %, and 14 volume
%.
(Preparation of Organic EL elements 2-3 to 2-9)
Organic EL elements 2-3 to 2-9 were prepared in the same manner as
preparation of the organic EL element 2-2 except that the third
compound was changed as indicated in Table 2.
TABLE-US-00002 TABLE 2 Emission Organic EL Third Angle .theta.
.DELTA.E.sub.ST efficiency element No. component (degree) (eV) (%)
Remarks 2-1 None -- -- 100 Comp. 2-2 Comparative 50 0.47 105 Comp.
compound 1 2-3 T-71 130 0.18 115 Inv. 2-4 T-19 135 0.02 119 Inv.
2-5 T-2 120 0.04 118 Inv. 2-6 T-86 141 0.03 125 Inv. 2-7 T-93 164
0.01 130 Inv. 2-8 Comparative 71 0.07 108 Comp. compound 2 2-9
T-110 120 0.01 112 Inv. Comp.: Comparative example Inv.: Inventive
example
Example 3
(Preparation of Organic EL Element 3-1)
An anode was prepared on a glass substrate of 50 mm.times.50 mm
with a thickness of 0.7 mm by forming a film of ITO (indium tin
oxide) with a thickness of 150 nm, then by making patterning to it.
The transparent support substrate provided with the ITO transparent
electrode was subjected to ultrasonic washing with isopropyl
alcohol, followed by drying with desiccated nitrogen gas, and it
was subjected to UV ozone washing for 5 minutes. The transparent
support substrate was fixed to a substrate holder of a commercial
vacuum deposition apparatus.
The constituting materials for each layer were loaded in each
heating boat for vapor deposition in the vacuum deposition
apparatus with an optimum amount. As a heating boat for vapor
deposition, it was used a resistance heating boat made of
molybdenum or tungsten.
After reducing the pressure of a vacuum tank to 4.times.10.sup.-4
Pa, the heating boat containing HAT-CN
(1,4,5,8,9,12-hexaazatriphenylene hexacarbonitrile) was heated via
application of electric current and deposition was made onto the
ITO transparent electrode at a deposition rate of 0.1 nm/second,
whereby it was produced a hole injection layer having a thickness
of 15 nm.
Subsequently, .alpha.-NPD
(4,4'-bis[N-(naphthyl)-N-phenylamino]biphenyl) was deposited onto
the hole injection layer at a deposition rate of 0.1 nm/second,
whereby it was produced a hole transport layer having a thickness
of 30 nm.
Subsequently, the heating boats each containing the comparative
compound 1 as a host compound and tris (2-phenylpyridinate) iridium
(III) were heated via application of electric current and
co-deposition was made onto the hole transport layer at a
deposition rate of 0.1 nm/second and 0.010 nm/second, whereby it
was produced a light emitting layer having a thickness of 40
nm.
Subsequently, HB-1 was deposited at a deposition rate of 0.1
nm/second, whereby it was produced a first electron transport layer
having a thickness of 5 nm.
##STR00027##
Further, ET-1 was deposited thereon at a deposition rate of 0.1
nm/second, whereby it was produced a second electron transport
layer having a thickness of 45 nm.
##STR00028##
Further, after forming a lithium fluoride layer having a thickness
of 0.5 nm, 100 nm thick aluminum was vapor deposited to forma
cathode. Thus, an organic EL element 3-1 was prepared.
(Preparation of Organic EL Elements 3-2 to 3-7)
Organic EL elements 3-2 to 3-7 were prepared in the same manner as
preparation of the organic EL element 3-1 except that the host
compound was changed as indicated in Table 3.
In the same manner as described above, an emission luminance of the
organic EL element 3-1 was measured. A relative emission luminance
of each organic EL element was obtained with respect to the
emission luminance of the organic EL element 3-1. The obtained
measurement results are listed in Table 3.
TABLE-US-00003 TABLE 3 Emission Organic EL Host Angle .theta.
.DELTA.E.sub.ST efficiency element No. component (degree) (eV) (%)
Remarks 3-1 Comparative 50 0.47 100 Comp. Compound 1 3-2
Comparative 71 0.07 87 Comp. compound 2 3-3 T-71 130 0.18 115 Inv.
3-4 T-86 141 0.03 118 Inv. 3-5 T-93 164 0.01 123 Inv. 3-6 T-97 142
0.18 117 Inv. 3-7 T-101 110 0.01 109 Inv. Comp.: Comparative
example Inv.: Inventive example
(Measurement of Emission Efficiency)
Emission efficiency of an organic EL element sample during driving
was evaluated by conducting the following measurement.
(Measurement of Emission Efficiency)
Each organic EL element thus produced was allowed to emit light by
applying a constant electric current of 2.5 mA/cm.sup.2 at room
temperature (about 25.degree. C.). The emission luminance
immediately after starting to emit light was measured with
Spectroradiometer CS-2000 (produced by Konica Minolta, Inc.). The
emission efficiency was determined. The obtained results were
indicated as a relative value in Tables 1 to 3.
In Example 1, the indicated values were a relative value when the
emission efficiency of the organic EL element 1-1 was set to be
100%. In Example 2, the indicated values were a relative value when
the emission efficiency of the organic EL element 2-1 was set to be
100%. Further, in Example 3, the indicated values were a relative
value when the emission efficiency of the organic EL element 3-1
was set to be 100%.
CONCLUSION
An absolute value of the difference between the first (lowest)
singlet excited level and the first (lowest) triplet excited level
(.DELTA.E.sub.ST) was calculated based on an optimized structure
for calculation of an angle .theta.. The calculation of the excited
state was done using Time-dependent density-functional calculation
method (DFT) with Gaussian 09 using B3LYP as a functional and
6-31G(d) as a base function.
In Table 1, the organic EL elements 1-2 to 1-8 had a larger .theta.
value and a smaller .DELTA.E.sub.ST value compared with the organic
EL elements 1-1 and 1-9. It was shown that the organic EL elements
1-2 to 1-8 exhibited improved emission efficiency.
In Table 2, it was shown that the organic EL elements 2-2 and 2-8
containing a third component exhibited improved emission efficiency
compared with the organic EL elements 2-1. It was shown that the
organic EL elements 2-3 to 2-7 and 2-9 containing a third component
having a larger angle .theta. exhibited further improved emission
efficiency compared with the organic EL elements 2-1.
In Table 3, it was shown that the organic EL elements 3-3 and 3-7
had a larger .theta. value and a smaller .DELTA.E.sub.ST value
compared with the organic EL elements 3-1 and 3-2. The organic EL
elements 3-3 and 3-7 exhibited improved emission efficiency.
INDUSTRIAL APPLICABILITY
As described above, the present invention is suitable to provide an
organic electroluminescent element enabling to achieve restrained
broadening of an absorption spectrum and an emission spectrum, and
high emission efficiency without using a rare metal.
DESCRIPTION OF SYMBOLS
1: Display 3: Pixel 5: Scanning line 6: Data line 7: Electric
source line 10: Organic EL element 11: Switching transistor 12:
Operating transistor 13: Capacitor 101: Organic EL element in a
light emitting device 102: Glass cover 105: Cathode 106: Organic EL
layer 107: Glass substrate having a transparent electrode 108:
Nitrogen gas 109: Water catching agent A: Display section B:
Control section C: Wiring section
* * * * *